On Wed 01-10-25 10:09:55, Jakub Acs wrote:

Calling intotify_show_fdinfo() on fd watching an overlayfs inode, while the overlayfs is being unmounted, can lead to dereferencing NULL ptr.

This issue was found by syzkaller.

Race Condition Diagram:

Thread 1 Thread 2

---

generic_shutdown_super() shrink_dcache_for_umount sb->s_root = NULL

                |
                |             vfs_read()
                |              inotify_fdinfo()
                |               * inode get from mark *
                |               show_mark_fhandle(m, inode)
                |                exportfs_encode_fid(inode, ..)
                |                 ovl_encode_fh(inode, ..)
                |                  ovl_check_encode_origin(inode)
                |                   * deref i_sb->s_root *
                |
                |
                v

fsnotify_sb_delete(sb)

Which then leads to:

[ 32.133461] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI [ 32.134438] KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] [ 32.135032] CPU: 1 UID: 0 PID: 4468 Comm: systemd-coredum Not tainted 6.17.0-rc6 #22 PREEMPT(none)

<snip registers, unreliable trace>

[ 32.143353] Call Trace: [ 32.143732] ovl_encode_fh+0xd5/0x170 [ 32.144031] exportfs_encode_inode_fh+0x12f/0x300 [ 32.144425] show_mark_fhandle+0xbe/0x1f0 [ 32.145805] inotify_fdinfo+0x226/0x2d0 [ 32.146442] inotify_show_fdinfo+0x1c5/0x350 [ 32.147168] seq_show+0x530/0x6f0 [ 32.147449] seq_read_iter+0x503/0x12a0 [ 32.148419] seq_read+0x31f/0x410 [ 32.150714] vfs_read+0x1f0/0x9e0 [ 32.152297] ksys_read+0x125/0x240

IOW ovl_check_encode_origin derefs inode->i_sb->s_root, after it was set to NULL in the unmount path.

Fix it by protecting calling exportfs_encode_fid() from show_mark_fhandle() with s_umount lock.

This form of fix was suggested by Amir in [1].

Fixes: c45beebfde34 ("ovl: support encoding fid from inode with no alias") Signed-off-by: Jakub Acs acsjakub@amazon.de Cc: Jan Kara jack@suse.cz Cc: Amir Goldstein amir73il@gmail.com Cc: Miklos Szeredi miklos@szeredi.hu Cc: Christian Brauner brauner@kernel.org Cc: linux-unionfs@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org

---

This issue was already discussed in [1] with no consensus reached on the fix.

This form was suggested as a band-aid fix, without explicity yes/no reaction. Hence reviving the discussion around the band-aid.

FWIW I'm working on a proper fix. But it's a larger rework so it will take some time to settle. For the time being, since this seems to happen in practical workloads, I guess we can live with this workaround so I'll pick this patch, add some comment about band-aid into the code and push it to Linus. Thanks!

Honza

fs/notify/fdinfo.c | 6 ++++++ 1 file changed, 6 insertions(+)

diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c index 1161eabf11ee..9cc7eb863643 100644 --- a/fs/notify/fdinfo.c +++ b/fs/notify/fdinfo.c @@ -17,6 +17,7 @@ #include "fanotify/fanotify.h" #include "fdinfo.h" #include "fsnotify.h" +#include "../internal.h" #if defined(CONFIG_PROC_FS) @@ -46,7 +47,12 @@ static void show_mark_fhandle(struct seq_file *m, struct inode *inode) size = f->handle_bytes >> 2;

• if (!super_trylock_shared(inode->i_sb))

• return;
  

• ret = exportfs_encode_fid(inode, (struct fid *)f->f_handle, &size);
• up_read(&inode->i_sb->s_umount);
• if ((ret == FILEID_INVALID) || (ret < 0)) return;

2.47.3

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