[v2] manual: Recommendations for dynamic linker hardening

This new section in the manual provides recommendations for
use of glibc in environments with higher integrity requirements.
It's reflecting both current implementation shortcomings, and
challenges we inherit from ELF and psABI requirements.

---
v2: Typos identified by Marek and Joe.  Clarify GNU2 TLS descriptors and
    dlopen, and relationship to security hardening.

 manual/dynlink.texi | 451 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 451 insertions(+)


base-commit: a07e000e82cb71238259e674529c37c12dc7d423

On 14/05/24 12:50 +0200, Florian Weimer wrote:
>This new section in the manual provides recommendations for
>use of glibc in environments with higher integrity requirements.
>It's reflecting both current implementation shortcomings, and
>challenges we inherit from ELF and psABI requirements.
>
>---
>v2: Typos identified by Marek and Joe.  Clarify GNU2 TLS descriptors and
>    dlopen, and relationship to security hardening.
>
> manual/dynlink.texi | 451 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 451 insertions(+)
>
>diff --git a/manual/dynlink.texi b/manual/dynlink.texi
>index d71f7a30d6..561b82665a 100644
>--- a/manual/dynlink.texi
>+++ b/manual/dynlink.texi
>@@ -15,6 +15,7 @@ Dynamic linkers are sometimes called @dfn{dynamic loaders}.
> @menu
> * Dynamic Linker Invocation::   Explicit invocation of the dynamic linker.
> * Dynamic Linker Introspection::    Interfaces for querying mapping information.
>+* Dynamic Linker Hardening::    Avoiding unexpected issues with dynamic linking.
> @end menu
>
> @node Dynamic Linker Invocation
>@@ -535,6 +536,456 @@ information is processed.
> This function is a GNU extension.
> @end deftypefun
>
>+@node Dynamic Linker Hardening
>+@section Avoiding Unexpected Issues With Dynamic Linking
>+
>+This section details recommendations for increasing application
>+robustness, by avoiding potential issues related to dynamic linking.
>+The recommendations have two main aims: reduce the involvement of the
>+dynamic linker in application execution after process startup, and
>+restrict the application to a dynamic linker feature set whose behavior
>+is more easily understood.
>+
>+Key aspects of limiting dynamic linker usage after startup are: no use
>+of the @code{dlopen} function, disabling lazy binding, and using the
>+static TLS model.  More easily understood dynamic linker behavior
>+requires avoiding name conflicts (symbols and sonames) and highly
>+customizable features like the audit subsystem.
>+
>+Note that while these steps can be considered a form of application
>+hardening, they do not guard against potential harm from accidental or
>+deliberate loading of untrusted or malicious code.  There is only
>+limited overlap with traditional security hardening for applications
>+running on GNU systems.
>+
>+@subsection Restricted Dynamic Linker Features
>+
>+Avoiding certain dynamic linker features can increase predictability of
>+applications and reduce the risk of running into dynamic linker defects.
>+
>+@itemize @bullet
>+@item
>+Do not use the functions @code{dlopen}, @code{dlmopen}, or
>+@code{dlclose}.  Dynamic loading and unloading of shared objects
>+introduces substantial complications related to symbol and thread-local
>+storage (TLS) management.
>+
>+@item
>+Without the @code{dlopen} function, @code{dlsym} and @code{dlvsym}
>+cannot be used with shared object handles.  Minimizing the use of both
>+functions is recommended.  If they have to be used, only the
>+@code{RTLD_DEFAULT} pseudo-handle should be used.
>+
>+@item
>+Use the local-exec or initial-exec TLS models.  If @code{dlopen} is not
>+used, there are no compatibility concerns for initial-exec TLS.  This
>+TLS model avoids most of the complexity around TLS access.  In
>+particular, there are no run-time memory allocations after process or
>+thread start.
>+
>+If shared objects are expected to be used more generally, outside the
>+restricted context, lack of compatibility between @code{dlopen} and
>+initial-exec TLS could be a concern.  In that case, the second-best
>+alternative is to use global-dynamic TLS with GNU2 TLS descriptors, for
>+targets that fully implement them, including the fast path for access to
>+TLS variables defined in the initially loaded set of objects.  Like
>+initial-exec TLS, this avoids memory allocations after thread creation,
>+but only if the @code{dlopen} function is not used.
>+
>+@item
>+Do not use lazy binding.  Lazy binding may require run-time memory
>+allocation, is not async-signal-safe, and introduces considerable
>+complexity.
>+
>+@item
>+Limit the use of indirect function (IFUNC) resolvers.  These resolvers
>+run during relocation processing, when @theglibc{} is not in a fully
>+consistent state.  If you use IFUNC resolvers, do not depend on external
>+data or function references.
>+
>+@item
>+Do not use the audit functionality (@code{LD_AUDIT}, @code{DT_AUDIT},
>+@code{DT_DEPAUDIT}).  Its callback and hooking capabilities introduce a
>+lot of complexity and subtly alter dynamic linker behavior in corner
>+cases even if the audit module is inactive.
>+
>+@item
>+Do not use symbol interposition, except in cases where copy relocations
>+are involved.  Without symbol interposition, the exact order in which
>+shared objects are searched are less relevant.
>+
>+@item
>+One potential source of symbol interposition is a combination of static
>+and dynamic linking, namely linking a static archive into multiple
>+dynamic shared objects.  For such scenarios, the static library should
>+be converted into its own dynamic shared object.
>+
>+A different approach to this situation uses hidden visibility for
>+symbols in the static library, but this can cause problems if the
>+library does not expect that multiple copies of its code coexist within
>+the same process, with no or partial sharing of state.
>+
>+@item
>+If you use shared objects that are linked with @option{-Wl,-Bsymbolic}
>+(or equivalent) or use protected visibility, the code for the main
>+program must be built as @option{-fpic} to avoid creating copy
>+relocations (and the main program must not use copy relocations for
>+other reasons).
>+
>+@item
>+Be careful about explicit section annotations.  Make sure that the
>+target section matches the properties of the declared entity (e.g., no
>+writable objects in @code{.text}).
>+
>+@item
>+Ensure that all assembler or object input files have the recommended
>+security markup, particularly for non-executable stack.
>+
>+@item
>+Some features of @theglibc{} indirectly depend on run-time code loading
>+and @code{dlopen}.  Use @code{iconv_open} with built-in converters only
>+(such as @code{UTF-8}).  Do not use NSS functionality such as
>+@code{getaddrinfo} or @code{getpwuid_r} unless the system is configured
>+for built-in NSS service modules only (see below).
>+@end itemize
>+
>+Several considerations apply to ELF constructors and destructors.
>+
>+@itemize @bullet
>+@item
>+The dynamic linker does not take constructor and destructor priorities
>+into account when determining their execution order.  Priorities are
>+only used by the link editor for ordering execution within a completely
>+linked object.
>+
>+@item
>+The recommendations to avoid cyclic dependencies and symbol

Where is the recommendation to avoid cyclic dependencies?

>+interposition (and the next two recommendations) make it less likely
>+that ELF objects are accessed before their ELF constructors have run (or
>+after their ELF destructors have run).  However, using @code{dlsym} and
>+@code{dlvsym}, it is still possible to access uninitialized facilities
>+even with these restrictions in place.  Such access is also possible
>+within a single shared object (or the main executable).  Consider using
>+dynamic, on-demand initialization instead.  To deal with access after
>+de-initialization, it may be necessary to implement special cases for
>+that scenario, potentially with degraded functionality.
>+
>+@item
>+Do not use the @code{DT_PREINIT_ARRAY} dynamic tag.
>+
>+@item
>+Do not flag objects as @code{DF_1_INITFIRST}.
>+
>+@item
>+If @code{dlopen} and @code{dlmopen} are not used, @code{DT_NEEDED}
>+dependency information is complete, and lazy binding is disabled, the
>+execution order of ELF destructors is expected to be the reverse of the
>+ELF constructor order.  However, two separate dependency sort operations
>+still occur.  Even though the listed preconditions should ensure that
>+both sorts produce the same ordering, it is recommended not to depend on
>+the destructor order being the reverse of the constructor order.
>+@end itemize
>+
>+The following items provide C++-specific guidance for preparing
>+applications.  If another programming language is used and it uses these
>+toolchain features targeted at C++ to implement some language
>+constructs, these restrictions and recommendations still apply in
>+analogous ways.
>+
>+@itemize @bullet
>+@item
>+C++ inline functions, templates, and other constructs may need to be
>+duplicated into multiple shared objects, resulting in symbol
>+interposition.  (This scenario is similar to combining static and
>+dynamic linking, as discussed above.)
>+
>+It may be possible to use explicit template instantiations to avoid
>+symbol interposition arising from multiple copies of the same template.
>+Hidden visibility could be used as well, but this may cause problems if
>+the duplicated functionality cannot deal with multiple copies within the
>+same process, with no or only partial state sharing.
>+
>+In general, due to the way certain C++ features are implemented in the
>+toolchain, avoiding symbol interposition entirely may not be possible as
>+long as these C++ features are used.
>+
>+@item
>+The toolchain and dynamic linker have multiple mechanisms that bypass
>+the usual symbol binding procedures.  This means that the C++ one
>+definition rule (ODR) still holds even if certain symbol-based isolation
>+mechanisms are used, and object addresses are not shared across
>+translation units with incompatible type definitions.  This does not
>+matter if the previous advice regarding symbol interposition is
>+followed.  However, as the advice may be difficult to implement, it is
>+necessary to avoid ODR violations across the entire process image.
>+
>+@item
>+Be aware that as a special of interposed symbols, symbols with the

s/as a special/as a special case/ ?

>+@code{STB_GNU_UNIQUE} type do not follow the usual symbol namespace
>+isolation rules: such symbols bind across @code{RTLD_LOCAL} and
>+@code{dlmopen} boundaries.  Furthermore, symbol versioning is ignored
>+for such symbols; they are bound by symbol name only.  All their
>+definitions and uses must therefore be compatible.  Hidden visibility
>+still prevents the creation of @code{STB_GNU_UNIQUE} symbols and can
>+achieve isolation of incompatible definitions.

This still violates the ODR though, right? Would it be worth appending
to the last sentence something like "at the risk of violating the
ODR"?

Or does using hidden visibility (which is outside the scope of the
C++ standard) make them distinct definitions which are then not
required to be identical?

>+
>+@item
>+C++ exception handling and run-time type information (RTTI), as
>+implemented in the GNU toolchain, is not address-significant, and
>+therefore is not affected by the symbol binding behaviour of the dynamic
>+linker.  This means that types of the same fully-qualified name (in
>+non-anonymous namespaces) are always considered the same from an
>+exception-handling or RTTI perspective.  This is true if the type

s/this is true if/this is true even if/ ?

>+information object or vtable has hidden symbol visibility, or the
>+corresponding symbols are versioned under different symbol versions, or
>+the symbols not bound to the same objects due to the use of

s/symbols not bound/symbols are not bound/ ?

>+@code{RTLD_LOCAL} or @code{dlmopen}.
>+
>+This can cause issues in applications that contain multiple incompatible
>+definitions of the same type.

Which is why the ODR disallows it.

>+@item
>+C++ exception handling across multiple @code{dlmopen} namespaces may
>+not work, particular with the unwinder in GCC versions before 12.
>+Current toolchain versions are able to process unwinding tables across
>+@code{dlmopen} boundaries.  However, note that type comparison is
>+name-based, not address-based (see the previous item), so exception
>+types may still be matched in unexpected ways.  An important special
>+case of exception handling, invoking destructors for variables of block
>+scope, is not impacted by this RTTI type-sharing.  Likewise, regular
>+virtual member function dispatch for objects is unaffected (but still
>+requires that the type definitions match in all directly involved
>+translation units).
>+
>+@item
>+The Itanium C++ ABI requires that in some cases, C++ destructors for
>+global objects that are constructed on demand are not destructed in the

s/destructed/destroyed/

Also, this sentence says "destructors [...] are not destructed" but it
means that the global objects are not destroyed, rather than the
destructors for the global objects.

>+opposite order of their construction.  (The C++ standard requires
>+opposite constructor order.)  As a result, do not depend on the precise

I had to read this a couple of times to decide if "opposite
constructor order" means something different from "opposite order of
their construction". I think the parenthesis could be made clearer, so
it's more obviously saying that the previous sentence describes a
deviation from the C++ standard. Maybe:

"Although the C++ standard requires that destructors for global
objects run in the opposite order of their constructors, the Itanium 
C++ ABI requires a different destruction order in some cases. As a
result, do not rely ..."

>+destructor invocation order.
>+
>+@item
>+Registering destructors for later invocation allocates memory and may
>+result in process termination if insufficient memory is available.  In
>+particular, this applies to dynamic initialization of a block variable
>+with static storage duration of a type that has a non-trivial destructor.
>+If unexpected program termination is a concern, ensure that such
>+objects merely have trivial destructors, avoiding the need for
>+registration.
>+
>+This includes @code{thread_local} objects with destructors.  Callbacks
>+for destruction of per-thread objects can be registered using
>+@code{pthread_key_create} in a way that permits handling memory
>+allocation errors.
>+@end itemize
>+
>+
>+@subsection Producing Matching Binaries
>+
>+This subsection recommends tools and build flags for producing
>+applications that meet the recommendations of the previous subsection.
>+
>+@itemize @bullet
>+@item
>+Use BFD ld (@command{bfd.ld}) from GNU binutils to produce binaries,
>+invoked through a compiler driver such as @command{gcc}.  The version
>+should be not too far ahead of what was current when the version of
>+@theglibc{} was first released.
>+
>+@item
>+Do not use a binutils release that is older than the one used to build
>+@theglibc{} itself.

Is the binutils version used to compile glibc readily available to
users who install glibc from their distro?

>+@item
>+Compile with @option{-ftls-model=initial-exec} to force the initial-exec
>+TLS model.
>+
>+@item
>+Link with @option{-Wl,-z,now} to disable lazy binding.
>+
>+@item
>+Link with @option{-Wl,-z,relro} to enable RELRO (which is the default on
>+most targets).
>+
>+@item
>+Specify all direct shared objects dependencies using @option{-l} options
>+to avoid underlinking.  Rely on @code{.so} files (which can be linker
>+scripts) and searching with the @option{-l} option.  Do not specify the
>+file names of shared objects on the linker command line.
>+
>+@item
>+Consider using @option{-Wl,-z,defs} to treat underlinking as an error
>+condition.
>+
>+@item
>+For a shared object (linked with @option{-shared}, use

Missing ')'

This might be clearer as "When creating a shared object ..."

>+@option{-Wl,-soname,lib@dots{}} to set a soname that matches the final
>+installed name of the file.
>+
>+@item
>+Do not use the @option{-rpath} linker option.

It's not clear from the preceding text why this option is a problem.

>+@item
>+Use @option{-Wl,--error-rwx-segments} and @option{-Wl,--error-execstack} to
>+instruct the link editor to fail the link if the resulting final object
>+would have read-write-execute segments or an executable stack.  Such
>+issues usually indicate that the input files are not marked up
>+correctly.
>+
>+@item
>+Ensure that for each @code{LOAD} segment in the ELF program header, file
>+offsets, memory sizes, and load addresses are multiples of the largest
>+page size supported at run time.  Similarly, the start address and size
>+of the @code{GNU_RELRO} range should be multiples of the page size.
>+
>+Avoid creating gaps between @code{LOAD} segments.  The difference
>+between the load addresses of two subsequent @code{LOAD} segments should
>+be the size of the first @code{LOAD} segment.  (This may require linking
>+with @option{-Wl,-z,noseparatecode}.)
>+
>+This may not be possible to achieve with the currently available link
>+editors.
>+
>+@item
>+If the multiple-of-page-size criterion for the @code{GNU_RELRO} region
>+cannot be achieved, ensure that the process memory image right before
>+the start of the region does not contain executable or writable memory.
>+@c https://sourceware.org/pipermail/libc-alpha/2022-May/138638.html
>+@end itemize
>+
>+@subsection Checking Binaries
>+
>+In some cases, if the previous recommendations are not followed, this
>+can be determined from the produced binaries.  This section contains
>+suggestions for verifying aspects of these binaries.
>+
>+@itemize @bullet
>+@item
>+To detect underlinking, examine the dynamic symbol table, for example
>+using @samp{readelf -sDW}.  If the symbol is defined in a shared object
>+that uses symbol versioning, it must carry a symbol version, as in
>+@samp{pthread_kill@@GLIBC_2.34}.
>+
>+@item
>+Examine the dynamic segment with @samp{readelf -dW} to check that all
>+the required @code{NEEDED} entries are present.  (It is not necessary to
>+list indirect dependencies.)
>+
>+@item
>+The @code{NEEDED} entries should not contain full path names including
>+slashes, only @code{sonames}.
>+
>+@item
>+For a further consistency check, collect all shared objects referenced
>+via @code{NEEDED} entries in dynamic segments, transitively, starting at
>+the main program.  Then determine their dynamic symbol tables (using
>+@samp{readelf -sDW}, for example).
>+
>+Ideally, every symbol should be defined at most once.
>+
>+If there are interposed data symbols, check if the single interposing
>+definition is in the main program.  In this case, there must be a copy
>+relocation for it.  (This only applies to targets with copy relocations.)
>+
>+Function symbols should never be interposed.
>+
>+@item
>+Using the previously collected @code{NEEDED} entries, check that the
>+dependency graph does not contain any cycles.
>+
>+@item
>+The dynamic segment should also mention @code{BIND_NOW} on the
>+@code{FLAGS} line or @code{NOW} on the @code{FLAGS_1} line (one is
>+enough).
>+
>+@item
>+For shared objects (not main programs), if the program header has a
>+@code{PT_TLS} segment, the dynamic segment (as shown by @samp{readelf
>+-dW}) should contain the @code{STATIC_TLS} flag on the @code{FLAGS}
>+line.
>+
>+If @code{STATIC_TLS} is missing in shared objects, ensure that the
>+appropriate relocations for GNU2 TLS descriptors are used (for example,
>+@code{R_AARCH64_TLSDESC} or @code{R_X86_64_TLSDESC}).
>+
>+@item
>+There should not be a reference to the @code{__tls_get_addr} symbol in
>+the dynamic symbol table (in the @samp{readelf -sDW} output).
>+Thread-local storage must be accessed using the initial-exec (static)
>+model, or using GNU2 TLS descriptors.
>+
>+@item
>+Likewise, the functions @code{dlopen}, @code{dlmopen}, @code{dlclose}
>+should not be referenced from the dynamic symbol table.
>+
>+@item
>+For shared objects, there should be a @code{SONAME} entry that matches
>+the file name (the base name, i.e., the part after the slash).  The
>+@code{SONAME} string must not contain a slash @samp{/}.
>+
>+@item
>+For all objects, the dynamic segment (as shown by @samp{readelf -dW})
>+should not contain @code{RPATH} or @code{RUNPATH} entries.
>+
>+@item
>+Likewise, the dynamic segment should not show any @code{AUDIT},
>+@code{DEPAUDIT}, @code{AUXILIARY}, @code{FILTER}, or
>+@code{PREINIT_ARRAY} tags.
>+
>+@item
>+The @code{DF_1_INITFIRST} flag should not be used.
>+
>+@item
>+The program header must not have @code{LOAD} segments that are writable
>+and executable at the same time.
>+
>+@item
>+All produced objects should have @code{GNU_STACK} program header that is

s/should have/should have a/

>+not marked as executable.  (However, on some newer targets, a
>+non-executable stack is the default, so the @code{GNU_STACK} program
>+header is not required.)
>+@end itemize
>+
>+@subsection Run-time Considerations
>+
>+@itemize @bullet
>+@item
>+Install shared objects using their sonames in a default search path
>+directory (usually @file{/usr/lib64}).  Do not use symbolic links.
>+@c This is currently not standard practice.
>+
>+@item
>+The default search path must not contain objects with duplicate file
>+names or sonames.
>+
>+@item
>+Do not use environment variables (@code{LD_@dots{} variables such as
>+@code{LD_PRELOAD} or @code{LD_LIBRARY_PATH}}, or @code{GLIBC_TUNABLES})
>+to change default dynamic linker behavior.
>+
>+@item
>+Do not install shared objects in non-default locations.  (Such locations
>+are listed explicitly in the configuration file for @command{ldconfig},
>+usually @file{/etc/ld.so.conf}, or in files included from there.)
>+
>+@item
>+Do not configure dynamically-loaded NSS service modules, to avoid
>+accidental internal use of the @code{dlopen} facility.  The @code{files}
>+and @code{dns} modules are built in and do not rely on @code{dlopen}.
>+
>+@item
>+Do not truncate and overwrite files containing programs and shared
>+objects in place, while they are used.  Instead, write the new version
>+to a different path and use @code{rename} to replace the
>+already-installed version.
>+
>+@item
>+Be aware that during an update procedure that involves multiple object
>+files (shared objects and main programs), concurrently starting
>+processes may observe an inconsistent combination of object files (some
>+already updated, some still at the previous version).
>+@end itemize
>+
>
> @c FIXME these are undocumented:
> @c dladdr
>
>base-commit: a07e000e82cb71238259e674529c37c12dc7d423
>