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| author | Kees Cook <keescook@chromium.org> | 2012-12-20 15:05:16 -0800 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2013-01-11 09:18:17 -0800 |
| commit | 21838dbf96ce1e94d1ba80fdd491f9a0a6352ff1 (patch) | |
| tree | 8878db64b9fdcf5566018fda502a34540548b11b /fs/exec.c | |
| parent | cae5b83f40f584969767996e294a0cd322856c8b (diff) | |
| download | lwn-21838dbf96ce1e94d1ba80fdd491f9a0a6352ff1.tar.gz lwn-21838dbf96ce1e94d1ba80fdd491f9a0a6352ff1.zip | |
exec: do not leave bprm->interp on stack
commit b66c5984017533316fd1951770302649baf1aa33 upstream.
If a series of scripts are executed, each triggering module loading via
unprintable bytes in the script header, kernel stack contents can leak
into the command line.
Normally execution of binfmt_script and binfmt_misc happens recursively.
However, when modules are enabled, and unprintable bytes exist in the
bprm->buf, execution will restart after attempting to load matching
binfmt modules. Unfortunately, the logic in binfmt_script and
binfmt_misc does not expect to get restarted. They leave bprm->interp
pointing to their local stack. This means on restart bprm->interp is
left pointing into unused stack memory which can then be copied into the
userspace argv areas.
After additional study, it seems that both recursion and restart remains
the desirable way to handle exec with scripts, misc, and modules. As
such, we need to protect the changes to interp.
This changes the logic to require allocation for any changes to the
bprm->interp. To avoid adding a new kmalloc to every exec, the default
value is left as-is. Only when passing through binfmt_script or
binfmt_misc does an allocation take place.
For a proof of concept, see DoTest.sh from:
http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'fs/exec.c')
| -rw-r--r-- | fs/exec.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/fs/exec.c b/fs/exec.c index 0039055b1fc6..c6e6de423e73 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1175,9 +1175,24 @@ void free_bprm(struct linux_binprm *bprm) mutex_unlock(¤t->signal->cred_guard_mutex); abort_creds(bprm->cred); } + /* If a binfmt changed the interp, free it. */ + if (bprm->interp != bprm->filename) + kfree(bprm->interp); kfree(bprm); } +int bprm_change_interp(char *interp, struct linux_binprm *bprm) +{ + /* If a binfmt changed the interp, free it first. */ + if (bprm->interp != bprm->filename) + kfree(bprm->interp); + bprm->interp = kstrdup(interp, GFP_KERNEL); + if (!bprm->interp) + return -ENOMEM; + return 0; +} +EXPORT_SYMBOL(bprm_change_interp); + /* * install the new credentials for this executable */ |