* [PATCH 6.17 061/184] fs/notify: call exportfs_encode_fid with s_umount
[not found] <20251027183514.934710872@linuxfoundation.org>
@ 2025-10-27 18:35 ` Greg Kroah-Hartman
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From: Greg Kroah-Hartman @ 2025-10-27 18:35 UTC (permalink / raw)
To: stable
Cc: Greg Kroah-Hartman, patches, Jakub Acs, Jan Kara, Amir Goldstein,
Miklos Szeredi, Christian Brauner, linux-unionfs, linux-fsdevel,
linux-kernel
6.17-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jakub Acs <acsjakub@amazon.de>
commit a7c4bb43bfdc2b9f06ee9d036028ed13a83df42a upstream.
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].
[1]: https://lore.kernel.org/all/CAOQ4uxhbDwhb+2Brs1UdkoF0a3NSdBAOQPNfEHjahrgoKJpLEw@mail.gmail.com/
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
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/notify/fdinfo.c | 6 ++++++
1 file changed, 6 insertions(+)
--- 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
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;
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