Message ID | 20220127212734.218900-1-vt@altlinux.org |
---|---|
State | New |
Headers | show |
Series | 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread | expand |
On Donnerstag, 27. Januar 2022 22:27:34 CET Vitaly Chikunov wrote: > `struct dirent' returned from readdir(3) could be shorter than > `sizeof(struct dirent)', thus memcpy of sizeof length will overread > into unallocated page causing SIGSEGV. Example stack trace: It could be shorter or longer. Your patch is fixing both possibilities. I just realized BTW that there are other places that should be fixed as well. :/ For instance hw/9pfs/9p-synth.c (used for the 9p test cases [1][2]) is copying dirents, too. And in hw/9pfs/9p.c v9fs_do_readdir_with_stat() used by 9p2000.u doesn't even bother to copy at all, which is undefined behaviour as fs driver is running on a background thread and the dirent pointer might have mutated in the meantime with the next readdir() call. [1] https://wiki.qemu.org/Documentation/9p#Test_Cases [2] https://wiki.qemu.org/Documentation/9p#9p_Filesystem_Drivers > #0 0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + > 0x497eed) #1 0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 > + 0x4982e9) #2 0x0000555555eb7983 coroutine_trampoline > (/usr/bin/qemu-system-x86_64 + 0x963983) #3 0x00007ffff73e0be0 n/a (n/a + > 0x0) > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841 > Signed-off-by: Vitaly Chikunov <vt@altlinux.org> Adding qemu-stable on CC for making sure this patch will be handled for the stable branches as well. > --- > Tested on x86-64 Linux. > > hw/9pfs/codir.c | 7 +++++-- > include/qemu/osdep.h | 6 ++++++ > util/osdep.c | 23 +++++++++++++++++++++++ > 3 files changed, 34 insertions(+), 2 deletions(-) > > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c > index 032cce04c4..ea7f5e6578 100644 > --- a/hw/9pfs/codir.c > +++ b/hw/9pfs/codir.c > @@ -143,8 +143,11 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState > *fidp, } else { > e = e->next = g_malloc0(sizeof(V9fsDirEnt)); > } > - e->dent = g_malloc0(sizeof(struct dirent)); > - memcpy(e->dent, dent, sizeof(struct dirent)); > + e->dent = qemu_dirent_dup(dent); That's the actual fix. > + if (!e->dent) { > + err = -errno; > + break; > + } e->dent is never NULL, so this check can be removed. See explanation about g_malloc() below. > > /* perform a full stat() for directory entry if requested by caller > */ if (dostat) { > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > index d1660d67fa..b54d22db04 100644 > --- a/include/qemu/osdep.h > +++ b/include/qemu/osdep.h > @@ -805,6 +805,12 @@ static inline int > platform_does_not_support_system(const char *command) } > #endif /* !HAVE_SYSTEM_FUNCTION */ > > +/** > + * Actual 'struct dirent' size may be bigger or shorter than > + * sizeof(struct dirent) in many cases. > + */ > +struct dirent *qemu_dirent_dup(struct dirent *dent); > + I think this issue deserves a more verbose API doc comment, something like: /** * Duplicate directory entry @dent. * * It is highly recommended to use this function instead of open coding * duplication of @c dirent objects, because the actual @c struct @c dirent * size may be bigger or shorter than @c sizeof(struct dirent) and correct * handling is platform specific (see gitlab issue #841). * * @dent - original directory entry to be duplicated * @returns duplicated directory entry which should be freed with g_free() */ struct dirent *qemu_dirent_dup(struct dirent *dent); > #ifdef __cplusplus > } > #endif > diff --git a/util/osdep.c b/util/osdep.c > index 42a0a4986a..e39d1071fd 100644 > --- a/util/osdep.c > +++ b/util/osdep.c > @@ -33,6 +33,7 @@ > extern int madvise(char *, size_t, int); > #endif > > +#include <dirent.h> Wouldn't that break Windows builds? > #include "qemu-common.h" > #include "qemu/cutils.h" > #include "qemu/sockets.h" > @@ -615,3 +616,25 @@ writev(int fd, const struct iovec *iov, int iov_cnt) > return readv_writev(fd, iov, iov_cnt, true); > } > #endif > + > +struct dirent * > +qemu_dirent_dup(struct dirent *dent) > +{ > + struct dirent *dst; > +#if defined _DIRENT_HAVE_D_RECLEN > + /* Avoid use of strlen() if there's d_reclen. */ > + dst = g_malloc(dent->d_reclen); > +#else > + /* Fallback to a most portable way. */ > + const size_t reclen = offsetof(struct dirent, d_name) + > strlen(dent->d_name) + 1; + > + dst = g_malloc(reclen); > +#endif > + if (!dst) > + return NULL; > +#ifdef _DIRENT_HAVE_D_RECLEN > + return memcpy(dst, dent, dent->d_reclen); > +#else > + return memcpy(dst, dent, reclen); > +#endif > +} On the long run we probably should have a configure check whether d_reclen exists, but I would not insist on that now as there is a valid fallback solution at least. Note that g_malloc() never returns NULL, if it runs out of memory it terminates instead, so the NULL checks are unncessary: https://developer.gimp.org/api/2.0/glib/glib-Memory-Allocation.html#g-try-malloc Also I would prefer g_malloc0() over g_malloc(). Then by adding a variable for the d_reclen yes/no case, overall code can be reduced. So I would suggest something like this instead: struct dirent * qemu_dirent_dup(struct dirent *dent) { #if defined _DIRENT_HAVE_D_RECLEN /* Avoid use of strlen() if there's d_reclen. */ const size_t sz = dent->d_reclen; #else /* Fallback to a most portable way. */ const size_t sz = offsetof(struct dirent, d_name) + strlen(dent->d_name) + 1; #endif struct dirent *dst = g_malloc(sz); return memcpy(dst, dent, sz); } Best regards, Christian Schoenebeck
On Freitag, 28. Januar 2022 15:43:10 CET Christian Schoenebeck wrote: > Also I would prefer g_malloc0() over g_malloc(). Never mind about that one. g_malloc0() with immediate subsequent memcpy() and exact same size argument would be pointless. > Then by adding a variable for the d_reclen yes/no case, overall code can be > reduced. So I would suggest something like this instead: > > struct dirent * > qemu_dirent_dup(struct dirent *dent) > { > #if defined _DIRENT_HAVE_D_RECLEN > /* Avoid use of strlen() if there's d_reclen. */ > const size_t sz = dent->d_reclen; > #else > /* Fallback to a most portable way. */ > const size_t sz = offsetof(struct dirent, d_name) + > strlen(dent->d_name) + 1; > #endif > struct dirent *dst = g_malloc(sz); > return memcpy(dst, dent, sz); > } Best regards, Christian Schoenebeck
Christian, On Fri, Jan 28, 2022 at 03:43:10PM +0100, Christian Schoenebeck wrote: > On Donnerstag, 27. Januar 2022 22:27:34 CET Vitaly Chikunov wrote: > > `struct dirent' returned from readdir(3) could be shorter than > > `sizeof(struct dirent)', thus memcpy of sizeof length will overread > > into unallocated page causing SIGSEGV. Example stack trace: > > It could be shorter or longer. Your patch is fixing both possibilities. > > I just realized BTW that there are other places that should be fixed as well. > :/ > > For instance hw/9pfs/9p-synth.c (used for the 9p test cases [1][2]) is copying > dirents, too. I noticed this one too, it could theoretically result in overflow on CIFS if long filenames are prepared. But I decided to concentrate on this more important fix, since this happen on normal fs. I will apply all other your comments, except these new fixes. > And in hw/9pfs/9p.c v9fs_do_readdir_with_stat() used by 9p2000.u doesn't even > bother to copy at all, which is undefined behaviour as fs driver is running on > a background thread and the dirent pointer might have mutated in the meantime > with the next readdir() call. > > [1] https://wiki.qemu.org/Documentation/9p#Test_Cases > [2] https://wiki.qemu.org/Documentation/9p#9p_Filesystem_Drivers > > > #0 0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + > > 0x497eed) #1 0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 > > + 0x4982e9) #2 0x0000555555eb7983 coroutine_trampoline > > (/usr/bin/qemu-system-x86_64 + 0x963983) #3 0x00007ffff73e0be0 n/a (n/a + > > 0x0) > > > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841 > > Signed-off-by: Vitaly Chikunov <vt@altlinux.org> > > Adding qemu-stable on CC for making sure this patch will be handled for the > stable branches as well. > > > --- > > Tested on x86-64 Linux. > > > > hw/9pfs/codir.c | 7 +++++-- > > include/qemu/osdep.h | 6 ++++++ > > util/osdep.c | 23 +++++++++++++++++++++++ > > 3 files changed, 34 insertions(+), 2 deletions(-) > > > > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c > > index 032cce04c4..ea7f5e6578 100644 > > --- a/hw/9pfs/codir.c > > +++ b/hw/9pfs/codir.c > > @@ -143,8 +143,11 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState > > *fidp, } else { > > e = e->next = g_malloc0(sizeof(V9fsDirEnt)); > > } > > - e->dent = g_malloc0(sizeof(struct dirent)); > > - memcpy(e->dent, dent, sizeof(struct dirent)); > > + e->dent = qemu_dirent_dup(dent); > > That's the actual fix. > > > + if (!e->dent) { > > + err = -errno; > > + break; > > + } > > e->dent is never NULL, so this check can be removed. See explanation about > g_malloc() below. Yes, I forgot about this. > > > > /* perform a full stat() for directory entry if requested by caller > > */ if (dostat) { > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > > index d1660d67fa..b54d22db04 100644 > > --- a/include/qemu/osdep.h > > +++ b/include/qemu/osdep.h > > @@ -805,6 +805,12 @@ static inline int > > platform_does_not_support_system(const char *command) } > > #endif /* !HAVE_SYSTEM_FUNCTION */ > > > > +/** > > + * Actual 'struct dirent' size may be bigger or shorter than > > + * sizeof(struct dirent) in many cases. > > + */ > > +struct dirent *qemu_dirent_dup(struct dirent *dent); > > + > > I think this issue deserves a more verbose API doc comment, something like: > > /** > * Duplicate directory entry @dent. > * > * It is highly recommended to use this function instead of open coding > * duplication of @c dirent objects, because the actual @c struct @c dirent > * size may be bigger or shorter than @c sizeof(struct dirent) and correct > * handling is platform specific (see gitlab issue #841). > * > * @dent - original directory entry to be duplicated > * @returns duplicated directory entry which should be freed with g_free() > */ > struct dirent *qemu_dirent_dup(struct dirent *dent); > > > #ifdef __cplusplus > > } > > #endif > > diff --git a/util/osdep.c b/util/osdep.c > > index 42a0a4986a..e39d1071fd 100644 > > --- a/util/osdep.c > > +++ b/util/osdep.c > > @@ -33,6 +33,7 @@ > > extern int madvise(char *, size_t, int); > > #endif > > > > +#include <dirent.h> > > Wouldn't that break Windows builds? Possible, but I don't have windows build to test. What would you suggest as better fix? I took as example `util/path.c' which includes `dirent.h' too, but it is not handled anyway different than `util/osdep.c' > > > #include "qemu-common.h" > > #include "qemu/cutils.h" > > #include "qemu/sockets.h" > > @@ -615,3 +616,25 @@ writev(int fd, const struct iovec *iov, int iov_cnt) > > return readv_writev(fd, iov, iov_cnt, true); > > } > > #endif > > + > > +struct dirent * > > +qemu_dirent_dup(struct dirent *dent) > > +{ > > + struct dirent *dst; > > +#if defined _DIRENT_HAVE_D_RECLEN > > + /* Avoid use of strlen() if there's d_reclen. */ > > + dst = g_malloc(dent->d_reclen); > > +#else > > + /* Fallback to a most portable way. */ > > + const size_t reclen = offsetof(struct dirent, d_name) + > > strlen(dent->d_name) + 1; + > > + dst = g_malloc(reclen); > > +#endif > > + if (!dst) > > + return NULL; > > +#ifdef _DIRENT_HAVE_D_RECLEN > > + return memcpy(dst, dent, dent->d_reclen); > > +#else > > + return memcpy(dst, dent, reclen); > > +#endif > > +} > > On the long run we probably should have a configure check whether d_reclen > exists, but I would not insist on that now as there is a valid fallback > solution at least. > > Note that g_malloc() never returns NULL, if it runs out of memory it > terminates instead, so the NULL checks are unncessary: > https://developer.gimp.org/api/2.0/glib/glib-Memory-Allocation.html#g-try-malloc > > Also I would prefer g_malloc0() over g_malloc(). > > Then by adding a variable for the d_reclen yes/no case, overall code can be > reduced. So I would suggest something like this instead: > > struct dirent * > qemu_dirent_dup(struct dirent *dent) > { > #if defined _DIRENT_HAVE_D_RECLEN > /* Avoid use of strlen() if there's d_reclen. */ > const size_t sz = dent->d_reclen; > #else > /* Fallback to a most portable way. */ > const size_t sz = offsetof(struct dirent, d_name) + > strlen(dent->d_name) + 1; > #endif > struct dirent *dst = g_malloc(sz); > return memcpy(dst, dent, sz); > } > > Best regards, > Christian Schoenebeck Thanks! >
On Freitag, 28. Januar 2022 17:24:01 CET Vitaly Chikunov wrote: > > > diff --git a/util/osdep.c b/util/osdep.c > > > index 42a0a4986a..e39d1071fd 100644 > > > --- a/util/osdep.c > > > +++ b/util/osdep.c > > > @@ -33,6 +33,7 @@ > > > > > > extern int madvise(char *, size_t, int); > > > #endif > > > > > > +#include <dirent.h> > > > > Wouldn't that break Windows builds? > > Possible, but I don't have windows build to test. What would you > suggest as better fix? > > I took as example `util/path.c' which includes `dirent.h' too, but it is > not handled anyway different than `util/osdep.c' AFAICS the current expectation is to build QEMU for Windows with a POSIX-aware toolchain like msys2, cygwin or some cross compiler: https://wiki.qemu.org/Hosts/W32 So I suggest leave it as is, it should be fine. Otherwise we can still wrap it into something like #if !defined(WIN32) ... #endif later on if really required. Best regards, Christian Schoenebeck
diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c index 032cce04c4..ea7f5e6578 100644 --- a/hw/9pfs/codir.c +++ b/hw/9pfs/codir.c @@ -143,8 +143,11 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, } else { e = e->next = g_malloc0(sizeof(V9fsDirEnt)); } - e->dent = g_malloc0(sizeof(struct dirent)); - memcpy(e->dent, dent, sizeof(struct dirent)); + e->dent = qemu_dirent_dup(dent); + if (!e->dent) { + err = -errno; + break; + } /* perform a full stat() for directory entry if requested by caller */ if (dostat) { diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h index d1660d67fa..b54d22db04 100644 --- a/include/qemu/osdep.h +++ b/include/qemu/osdep.h @@ -805,6 +805,12 @@ static inline int platform_does_not_support_system(const char *command) } #endif /* !HAVE_SYSTEM_FUNCTION */ +/** + * Actual 'struct dirent' size may be bigger or shorter than + * sizeof(struct dirent) in many cases. + */ +struct dirent *qemu_dirent_dup(struct dirent *dent); + #ifdef __cplusplus } #endif diff --git a/util/osdep.c b/util/osdep.c index 42a0a4986a..e39d1071fd 100644 --- a/util/osdep.c +++ b/util/osdep.c @@ -33,6 +33,7 @@ extern int madvise(char *, size_t, int); #endif +#include <dirent.h> #include "qemu-common.h" #include "qemu/cutils.h" #include "qemu/sockets.h" @@ -615,3 +616,25 @@ writev(int fd, const struct iovec *iov, int iov_cnt) return readv_writev(fd, iov, iov_cnt, true); } #endif + +struct dirent * +qemu_dirent_dup(struct dirent *dent) +{ + struct dirent *dst; +#if defined _DIRENT_HAVE_D_RECLEN + /* Avoid use of strlen() if there's d_reclen. */ + dst = g_malloc(dent->d_reclen); +#else + /* Fallback to a most portable way. */ + const size_t reclen = offsetof(struct dirent, d_name) + strlen(dent->d_name) + 1; + + dst = g_malloc(reclen); +#endif + if (!dst) + return NULL; +#ifdef _DIRENT_HAVE_D_RECLEN + return memcpy(dst, dent, dent->d_reclen); +#else + return memcpy(dst, dent, reclen); +#endif +}
`struct dirent' returned from readdir(3) could be shorter than `sizeof(struct dirent)', thus memcpy of sizeof length will overread into unallocated page causing SIGSEGV. Example stack trace: #0 0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + 0x497eed) #1 0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 + 0x4982e9) #2 0x0000555555eb7983 coroutine_trampoline (/usr/bin/qemu-system-x86_64 + 0x963983) #3 0x00007ffff73e0be0 n/a (n/a + 0x0) Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841 Signed-off-by: Vitaly Chikunov <vt@altlinux.org> --- Tested on x86-64 Linux. hw/9pfs/codir.c | 7 +++++-- include/qemu/osdep.h | 6 ++++++ util/osdep.c | 23 +++++++++++++++++++++++ 3 files changed, 34 insertions(+), 2 deletions(-)