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exec: Add comments on check_unsafe_exec() fs counting

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Add some comments about what the fs counting is doing in
check_unsafe_exec() and how it relates to the call graph.
Specifically, we can't force an unshare of the fs because
of at least Chrome:
https://lore.kernel.org/lkml/86CE201B-5632-4BB7-BCF6-7CB2C2895409@chromium.org/

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Link: https://lore.kernel.org/r/20221018071537.never.662-kees@kernel.org
This commit is contained in:
Kees Cook 2022-10-18 00:17:24 -07:00
parent 23a7aea5fa
commit 275498a98b
1 changed files with 12 additions and 0 deletions
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12
fs/exec.c
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@ -1573,6 +1573,12 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
if (task_no_new_privs(current))
bprm->unsafe |= LSM_UNSAFE_NO_NEW_PRIVS;
/*
* If another task is sharing our fs, we cannot safely
* suid exec because the differently privileged task
* will be able to manipulate the current directory, etc.
* It would be nice to force an unshare instead...
*/
t = p;
n_fs = 1;
spin_lock(&p->fs->lock);
@ -1753,6 +1759,7 @@ static int search_binary_handler(struct linux_binprm *bprm)
return retval;
}
/* binfmt handlers will call back into begin_new_exec() on success. */
static int exec_binprm(struct linux_binprm *bprm)
{
pid_t old_pid, old_vpid;
@ -1811,6 +1818,11 @@ static int bprm_execve(struct linux_binprm *bprm,
if (retval)
return retval;
/*
* Check for unsafe execution states before exec_binprm(), which
* will call back into begin_new_exec(), into bprm_creds_from_file(),
* where setuid-ness is evaluated.
*/
check_unsafe_exec(bprm);
current->in_execve = 1;