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Message-ID: <20121119213005.GA2917@www.outflux.net>
Date: Mon, 19 Nov 2012 13:30:05 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-kernel@...r.kernel.org
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Josh Triplett <josh@...htriplett.org>,
Serge Hallyn <serge.hallyn@...onical.com>,
linux-fsdevel@...r.kernel.org, halfdog <me@...fdog.net>,
P J P <ppandit@...hat.com>
Subject: [PATCH] exec: use -ELOOP for max recursion depth
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.
Cc: halfdog <me@...fdog.net>
Cc: P J P <ppandit@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
fs/binfmt_em86.c | 1 -
fs/binfmt_misc.c | 6 ------
fs/binfmt_script.c | 4 +---
fs/exec.c | 10 +++++-----
include/linux/binfmts.h | 2 --
5 files changed, 6 insertions(+), 17 deletions(-)
diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index 2790c7e..575796a 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 772428d..f0f1a06 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -117,10 +117,6 @@ 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);
@@ -200,8 +196,6 @@ 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);
if (retval < 0)
goto _error;
diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index df49d48..8ae4be1 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 c6e6de4..85c1f9e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1371,6 +1371,10 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
struct linux_binfmt *fmt;
pid_t old_pid, old_vpid;
+ /* This allows 4 levels of binfmt rewrites before failing hard. */
+ if (depth > 5)
+ return -ELOOP;
+
retval = security_bprm_check(bprm);
if (retval)
return retval;
@@ -1395,12 +1399,8 @@ 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 de0628e..54135f6 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -54,8 +54,6 @@ 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 {
siginfo_t *siginfo;
--
1.7.9.5
--
Kees Cook
Chrome OS Security
--
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