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exec: Move handling of the point of no return to the top level

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Move the handing of the point of no return from search_binary_handler
into __do_execve_file so that it is easier to find, and to keep
things robust in the face of change.

Make it clear that an existing fatal signal will take precedence over
a forced SIGSEGV by not forcing SIGSEGV if a fatal signal is already
pending.  This does not change the behavior but it saves a reader
of the code the tedium of reading and understanding force_sig
and the signal delivery code.

Update the comment in begin_new_exec about where SIGSEGV is forced.

Keep point_of_no_return from being a mystery by documenting
what the code is doing where it forces SIGSEGV if the
code is past the point of no return.

Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/87y2q25knl.fsf_-_@x220.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-04-04 09:42:56 -05:00
parent a28bf136e6
commit 8890b29341
1 changed files with 12 additions and 9 deletions
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21
fs/exec.c
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@ -1329,8 +1329,8 @@ int begin_new_exec(struct linux_binprm * bprm)
/* /*
* With the new mm installed it is completely impossible to * With the new mm installed it is completely impossible to
* fail and return to the original process. If anything from * fail and return to the original process. If anything from
* here on returns an error, the check in * here on returns an error, the check in __do_execve_file()
* search_binary_handler() will SEGV current. * will SEGV current.
*/ */
bprm->point_of_no_return = true; bprm->point_of_no_return = true;
bprm->mm = NULL; bprm->mm = NULL;
@ -1721,13 +1721,8 @@ int search_binary_handler(struct linux_binprm *bprm)
read_lock(&binfmt_lock); read_lock(&binfmt_lock);
put_binfmt(fmt); put_binfmt(fmt);
if (retval < 0 && bprm->point_of_no_return) { if (bprm->point_of_no_return || !bprm->file ||
/* we got to flush_old_exec() and failed after it */ (retval != -ENOEXEC)) {
read_unlock(&binfmt_lock);
force_sigsegv(SIGSEGV);
return retval;
}
if (retval != -ENOEXEC || !bprm->file) {
read_unlock(&binfmt_lock); read_unlock(&binfmt_lock);
return retval; return retval;
} }
@ -1898,6 +1893,14 @@ static int __do_execve_file(int fd, struct filename *filename,
return retval; return retval;
out: out:
/*
* If past the point of no return ensure the the code never
* returns to the userspace process. Use an existing fatal
* signal if present otherwise terminate the process with
* SIGSEGV.
*/
if (bprm->point_of_no_return && !fatal_signal_pending(current))
force_sigsegv(SIGSEGV);
if (bprm->mm) { if (bprm->mm) {
acct_arg_size(bprm, 0); acct_arg_size(bprm, 0);
mmput(bprm->mm); mmput(bprm->mm);