Syzkaller detected a use-after-free issue in ext4_insert_dentry that was caused by out-of-bounds access due to incorrect splitting in do_split.
BUG: KASAN: use-after-free in ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 Write of size 251 at addr ffff888074572f14 by task syz-executor335/5847
CPU: 0 UID: 0 PID: 5847 Comm: syz-executor335 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024 Call Trace: <TASK> __dump_stack lib/dump_stack.c:94 [inline] dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:377 [inline] print_report+0x169/0x550 mm/kasan/report.c:488 kasan_report+0x143/0x180 mm/kasan/report.c:601 kasan_check_range+0x282/0x290 mm/kasan/generic.c:189 __asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106 ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 add_dirent_to_buf+0x3d9/0x750 fs/ext4/namei.c:2154 make_indexed_dir+0xf98/0x1600 fs/ext4/namei.c:2351 ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2455 ext4_add_nondir+0x8d/0x290 fs/ext4/namei.c:2796 ext4_symlink+0x920/0xb50 fs/ext4/namei.c:3431 vfs_symlink+0x137/0x2e0 fs/namei.c:4615 do_symlinkat+0x222/0x3a0 fs/namei.c:4641 __do_sys_symlink fs/namei.c:4662 [inline] __se_sys_symlink fs/namei.c:4660 [inline] __x64_sys_symlink+0x7a/0x90 fs/namei.c:4660 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK>
The following loop is located right above 'if' statement.
for (i = count-1; i >= 0; i--) { /* is more than half of this entry in 2nd half of the block? */ if (size + map[i].size/2 > blocksize/2) break; size += map[i].size; move++; }
'i' in this case could go down to -1, in which case sum of active entries wouldn't exceed half the block size, but previous behaviour would also do split in half if sum would exceed at the very last block, which in case of having too many long name files in a single block could lead to out-of-bounds access and following use-after-free.
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Cc: stable@vger.kernel.org Fixes: 5872331b3d91 ("ext4: fix potential negative array index in do_split()") Signed-off-by: Artem Sadovnikov a.sadovnikov@ispras.ru --- fs/ext4/namei.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c index cb5cb33b1d91..e9712e64ec8f 100644 --- a/fs/ext4/namei.c +++ b/fs/ext4/namei.c @@ -1971,7 +1971,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir, * split it in half by count; each resulting block will have at least * half the space free. */ - if (i > 0) + if (i >= 0) split = count - move; else split = count/2;
On Fri 04-04-25 08:28:05, Artem Sadovnikov wrote:
Syzkaller detected a use-after-free issue in ext4_insert_dentry that was caused by out-of-bounds access due to incorrect splitting in do_split.
BUG: KASAN: use-after-free in ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 Write of size 251 at addr ffff888074572f14 by task syz-executor335/5847
CPU: 0 UID: 0 PID: 5847 Comm: syz-executor335 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024 Call Trace:
<TASK> __dump_stack lib/dump_stack.c:94 [inline] dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:377 [inline] print_report+0x169/0x550 mm/kasan/report.c:488 kasan_report+0x143/0x180 mm/kasan/report.c:601 kasan_check_range+0x282/0x290 mm/kasan/generic.c:189 __asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106 ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 add_dirent_to_buf+0x3d9/0x750 fs/ext4/namei.c:2154 make_indexed_dir+0xf98/0x1600 fs/ext4/namei.c:2351 ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2455 ext4_add_nondir+0x8d/0x290 fs/ext4/namei.c:2796 ext4_symlink+0x920/0xb50 fs/ext4/namei.c:3431 vfs_symlink+0x137/0x2e0 fs/namei.c:4615 do_symlinkat+0x222/0x3a0 fs/namei.c:4641 __do_sys_symlink fs/namei.c:4662 [inline] __se_sys_symlink fs/namei.c:4660 [inline] __x64_sys_symlink+0x7a/0x90 fs/namei.c:4660 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK>
The following loop is located right above 'if' statement.
for (i = count-1; i >= 0; i--) { /* is more than half of this entry in 2nd half of the block? */ if (size + map[i].size/2 > blocksize/2) break; size += map[i].size; move++; }
'i' in this case could go down to -1, in which case sum of active entries wouldn't exceed half the block size, but previous behaviour would also do split in half if sum would exceed at the very last block, which in case of having too many long name files in a single block could lead to out-of-bounds access and following use-after-free.
Thanks for debugging this! The fix looks good, but I'm still failing to see the use-after-free / end-of-buffer issue. If we wrongly split to two parts count/2 each, then dx_move_dirents() and dx_pack_dirents() seem to still work correctly. Just they will make too small amount of space in bh but still at least one dir entry gets moved? Following add_dirent_to_buf() is more likely to fail due to ENOSPC but still I don't see the buffer overrun issue? Can you please tell me what I'm missing? Thanks!
Anyway, since the fix looks obviously correct feel free to add:
Reviewed-by: Jan Kara jack@suse.cz
Honza
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Cc: stable@vger.kernel.org Fixes: 5872331b3d91 ("ext4: fix potential negative array index in do_split()") Signed-off-by: Artem Sadovnikov a.sadovnikov@ispras.ru
fs/ext4/namei.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c index cb5cb33b1d91..e9712e64ec8f 100644 --- a/fs/ext4/namei.c +++ b/fs/ext4/namei.c @@ -1971,7 +1971,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir, * split it in half by count; each resulting block will have at least * half the space free. */
- if (i > 0)
- if (i >= 0) split = count - move; else split = count/2;
-- 2.43.0
On 07.04.2025 16:02, Jan Kara wrote:
Thanks for debugging this! The fix looks good, but I'm still failing to see the use-after-free / end-of-buffer issue. If we wrongly split to two parts count/2 each, then dx_move_dirents() and dx_pack_dirents() seem to still work correctly. Just they will make too small amount of space in bh but still at least one dir entry gets moved? Following add_dirent_to_buf() is more likely to fail due to ENOSPC but still I don't see the buffer overrun issue? Can you please tell me what I'm missing? Thanks!
add_dirent_to_buf() only checks for available space if its de parameter is NULL, but make_indexed_dir() provides a non-NULL de, so that space check is skipped entirely. add_dirent_to_buf() then calls ext4_insert_dentry() which will write a filename that's potentially larger than entry size and will cause an out-of-bounds write.
On Tue 08-04-25 16:38:36, Artem Sadovnikov wrote:
On 07.04.2025 16:02, Jan Kara wrote:
Thanks for debugging this! The fix looks good, but I'm still failing to see the use-after-free / end-of-buffer issue. If we wrongly split to two parts count/2 each, then dx_move_dirents() and dx_pack_dirents() seem to still work correctly. Just they will make too small amount of space in bh but still at least one dir entry gets moved? Following add_dirent_to_buf() is more likely to fail due to ENOSPC but still I don't see the buffer overrun issue? Can you please tell me what I'm missing? Thanks!
add_dirent_to_buf() only checks for available space if its de parameter is NULL, but make_indexed_dir() provides a non-NULL de, so that space check is skipped entirely. add_dirent_to_buf() then calls ext4_insert_dentry() which will write a filename that's potentially larger than entry size and will cause an out-of-bounds write.
Indeed. I didn't notice this detail. Thanks for explanation!
Honza
On Fri, 04 Apr 2025 08:28:05 +0000, Artem Sadovnikov wrote:
Syzkaller detected a use-after-free issue in ext4_insert_dentry that was caused by out-of-bounds access due to incorrect splitting in do_split.
BUG: KASAN: use-after-free in ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 Write of size 251 at addr ffff888074572f14 by task syz-executor335/5847
CPU: 0 UID: 0 PID: 5847 Comm: syz-executor335 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024 Call Trace:
<TASK> __dump_stack lib/dump_stack.c:94 [inline] dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:377 [inline] print_report+0x169/0x550 mm/kasan/report.c:488 kasan_report+0x143/0x180 mm/kasan/report.c:601 kasan_check_range+0x282/0x290 mm/kasan/generic.c:189 __asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106 ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 add_dirent_to_buf+0x3d9/0x750 fs/ext4/namei.c:2154 make_indexed_dir+0xf98/0x1600 fs/ext4/namei.c:2351 ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2455 ext4_add_nondir+0x8d/0x290 fs/ext4/namei.c:2796 ext4_symlink+0x920/0xb50 fs/ext4/namei.c:3431 vfs_symlink+0x137/0x2e0 fs/namei.c:4615 do_symlinkat+0x222/0x3a0 fs/namei.c:4641 __do_sys_symlink fs/namei.c:4662 [inline] __se_sys_symlink fs/namei.c:4660 [inline] __x64_sys_symlink+0x7a/0x90 fs/namei.c:4660 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK>
[...]
Applied, thanks!
[1/1] ext4: fix off-by-one error in do_split commit: 7e50bbb134aba1df0854f171b596b3a42d35605a
(Apologies for sending this late; I've been dealing with a family medical emergency. In any case, the patch landed in v6.16-rc2.)
Best regards,
linux-stable-mirror@lists.linaro.org
-
Artem Sadovnikov -
Jan Kara -
Theodore Ts'o