Re: [PATCH] kernel/fs: last check for exec credentials on NOEXEC mount

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Aug 01, 2024 at 02:07:45PM +0200, Wojciech Gładysz wrote:
> Test case: thread mounts NOEXEC fuse to a file being executed.
> WARN_ON_ONCE is triggered yielding panic for some config.
> Add a check to security_bprm_creds_for_exec(bprm).
> 

Need more detail here, a script or something to describe the series of events
that gets us here, I can't quite figure out how to do this.

> Stack trace:
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 2736 at fs/exec.c:933 do_open_execat+0x311/0x710 fs/exec.c:932
> Modules linked in:
> CPU: 0 PID: 2736 Comm: syz-executor384 Not tainted 5.10.0-syzkaller #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:do_open_execat+0x311/0x710 fs/exec.c:932
> Code: 89 de e8 02 b1 a1 ff 31 ff 89 de e8 f9 b0 a1 ff 45 84 ff 75 2e 45 85 ed 0f 8f ed 03 00 00 e8 56 ae a1 ff eb bd e8 4f ae a1 ff <0f> 0b 48 c7 c3 f3 ff ff ff 4c 89 f7 e8 9e cb fe ff 49 89 de e9 2d
> RSP: 0018:ffffc90008e07c20 EFLAGS: 00010293
> RAX: ffffffff82131ac6 RBX: 0000000000000004 RCX: ffff88801a6611c0
> RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000000
> RBP: ffffc90008e07cf0 R08: ffffffff8213173f R09: ffffc90008e07aa0
> R10: 0000000000000000 R11: dffffc0000000001 R12: ffff8880115810e0
> R13: dffffc0000000000 R14: ffff88801122c040 R15: ffffc90008e07c60
> FS:  00007f9e283ce6c0(0000) GS:ffff888058a00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f9e2848600a CR3: 00000000139de000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  bprm_execve+0x60b/0x1c40 fs/exec.c:1939
>  do_execveat_common+0x5a6/0x770 fs/exec.c:2077
>  do_execve fs/exec.c:2147 [inline]
>  __do_sys_execve fs/exec.c:2223 [inline]
>  __se_sys_execve fs/exec.c:2218 [inline]
>  __x64_sys_execve+0x92/0xb0 fs/exec.c:2218
>  do_syscall_64+0x6d/0xa0 arch/x86/entry/common.c:62
>  entry_SYSCALL_64_after_hwframe+0x61/0xcb
> RIP: 0033:0x7f9e2842f299
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 18 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f9e283ce218 EFLAGS: 00000246 ORIG_RAX: 000000000000003b
> RAX: ffffffffffffffda RBX: 00007f9e284bd3f8 RCX: 00007f9e2842f299
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000400
> RBP: 00007f9e284bd3f0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f9e2848a134
> R13: 0030656c69662f2e R14: 00007ffc819a23d0 R15: 00007f9e28488130
> 
> Signed-off-by: Wojciech Gładysz <wojciech.gladysz@infogain.com>
> ---
>  fs/exec.c | 42 +++++++++++++++++++-----------------------
>  1 file changed, 19 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index a126e3d1cacb..0cc6a7d033a1 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -953,8 +953,6 @@ EXPORT_SYMBOL(transfer_args_to_stack);
>   */
>  static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  {
> -	struct file *file;
> -	int err;
>  	struct open_flags open_exec_flags = {
>  		.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>  		.acc_mode = MAY_EXEC,
> @@ -969,26 +967,7 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	if (flags & AT_EMPTY_PATH)
>  		open_exec_flags.lookup_flags |= LOOKUP_EMPTY;
>  
> -	file = do_filp_open(fd, name, &open_exec_flags);
> -	if (IS_ERR(file))
> -		goto out;
> -
> -	/*
> -	 * may_open() has already checked for this, so it should be
> -	 * impossible to trip now. But we need to be extra cautious
> -	 * and check again at the very end too.
> -	 */
> -	err = -EACCES;
> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> -			 path_noexec(&file->f_path)))
> -		goto exit;
> -

This still needs to be left here to catch any bad actors in the future.  Thanks,

Josef