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[Oneiric,CVE] exec: use -ELOOP for max recursion depth

Message ID 1355920923-23703-4-git-send-email-luis.henriques@canonical.com
State New
Headers show

Commit Message

Luis Henriques Dec. 19, 2012, 12:42 p.m. UTC 
From: Kees Cook <keescook@chromium.org>

CVE-2012-4530

BugLink: http://bugs.launchpad.net/bugs/1068888

To avoid an explosion of request_module calls on a chain of abusive
scripts, fail maximum recursion with -ELOOP instead of -ENOEXEC. As soon
as maximum recursion depth is hit, the error will fail all the way back
up the chain, aborting immediately.

This also has the side-effect of stopping the user's shell from attempting
to reexecute the top-level file as a shell script. As seen in the
dash source:

        if (cmd != path_bshell && errno == ENOEXEC) {
                *argv-- = cmd;
                *argv = cmd = path_bshell;
                goto repeat;
        }

The above logic was designed for running scripts automatically that lacked
the "#!" header, not to re-try failed recursion. On a legitimate -ENOEXEC,
things continue to behave as the shell expects.

Additionally, when tracking recursion, the binfmt handlers should not be
involved. The recursion being tracked is the depth of calls through
search_binary_handler(), so that function should be exclusively responsible
for tracking the depth.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: halfdog <me@halfdog.net>
Cc: P J P <ppandit@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

(backported from commit d740269867021faf4ce38a449353d2b986c34a67)
Conflicts:
	fs/binfmt_misc.c
	fs/exec.c
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
 fs/binfmt_em86.c        |    1 -
 fs/binfmt_misc.c        |    8 +-------
 fs/binfmt_script.c      |    4 +---
 fs/exec.c               |   10 +++++-----
 include/linux/binfmts.h |    2 --
 5 files changed, 7 insertions(+), 18 deletions(-)
diff mbox

Patch

diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index b8e8b0a..4a1b984 100644
--- a/fs/binfmt_em86.c
+++ b/fs/binfmt_em86.c
@@ -42,7 +42,6 @@  static int load_em86(struct linux_binprm *bprm,struct pt_regs *regs)
 			return -ENOEXEC;
 	}
 
-	bprm->recursion_depth++; /* Well, the bang-shell is implicit... */
 	allow_write_access(bprm->file);
 	fput(bprm->file);
 	bprm->file = NULL;
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 5463952..82b4456 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -116,10 +116,6 @@  static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 	if (!enabled)
 		goto _ret;
 
-	retval = -ENOEXEC;
-	if (bprm->recursion_depth > BINPRM_MAX_RECURSION)
-		goto _ret;
-
 	/* to keep locking time low, we copy the interpreter string */
 	read_lock(&entries_lock);
 	fmt = check_file(bprm);
@@ -200,9 +196,7 @@  static int load_misc_binary(struct linux_binprm *bprm, struct pt_regs *regs)
 	if (retval < 0)
 		goto _error;
 
-	bprm->recursion_depth++;
-
-	retval = search_binary_handler (bprm, regs);
+	retval = search_binary_handler(bprm, regs);
 	if (retval < 0)
 		goto _error;
 
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index e39c18a..211ede0 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -22,15 +22,13 @@  static int load_script(struct linux_binprm *bprm,struct pt_regs *regs)
 	char interp[BINPRM_BUF_SIZE];
 	int retval;
 
-	if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!') ||
-	    (bprm->recursion_depth > BINPRM_MAX_RECURSION))
+	if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))
 		return -ENOEXEC;
 	/*
 	 * This section does the #! interpretation.
 	 * Sorta complicated, but hopefully it will work.  -TYT
 	 */
 
-	bprm->recursion_depth++;
 	allow_write_access(bprm->file);
 	fput(bprm->file);
 	bprm->file = NULL;
diff --git a/fs/exec.c b/fs/exec.c
index f7af22b..3558306 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1373,6 +1373,10 @@  int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
 	int try,retval;
 	struct linux_binfmt *fmt;
 
+	/* This allows 4 levels of binfmt rewrites before failing hard. */
+	if (depth > 5)
+		return -ELOOP;
+
 	retval = security_bprm_check(bprm);
 	if (retval)
 		return retval;
@@ -1391,12 +1395,8 @@  int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
 			if (!try_module_get(fmt->module))
 				continue;
 			read_unlock(&binfmt_lock);
+			bprm->recursion_depth = depth + 1;
 			retval = fn(bprm, regs);
-			/*
-			 * Restore the depth counter to its starting value
-			 * in this call, so we don't have to rely on every
-			 * load_binary function to restore it on return.
-			 */
 			bprm->recursion_depth = depth;
 			if (retval >= 0) {
 				if (depth == 0)
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 384e37f..718eb0b 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -67,8 +67,6 @@  struct linux_binprm {
 #define BINPRM_FLAGS_EXECFD_BIT 1
 #define BINPRM_FLAGS_EXECFD (1 << BINPRM_FLAGS_EXECFD_BIT)
 
-#define BINPRM_MAX_RECURSION 4
-
 /* Function parameter for binfmt->coredump */
 struct coredump_params {
 	long signr;