#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
syzbot reported use-after-free bugs when accessing extent headers in ext4_ext_insert_extent() and ext4_ext_correct_indexes(). These occur when the extent path structure becomes invalid during operations.
The crashes show two patterns: 1. In ext4_ext_map_blocks(), the extent header can be corrupted after ext4_find_extent() returns, particularly during concurrent writes to the same file. 2. In ext4_ext_correct_indexes(), accessing path[depth] causes a use-after-free, indicating the path structure itself is corrupted.
This is partially exposed by commit 665575cff098 ("filemap: move prefaulting out of hot write path") which changed timing windows in the write path, making these races more likely to occur.
Fix this by adding validation checks: - In ext4_ext_map_blocks(): validate the extent header after getting the path from ext4_find_extent() - In ext4_ext_correct_indexes(): validate the path pointer before dereferencing and check extent header magic
While these checks are defensive and don't address the root cause of path corruption, they prevent kernel crashes from invalid memory access. A more comprehensive fix to path lifetime management may be needed in the future.
Reported-by: syzbot+9db318d6167044609878@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=9db318d6167044609878 Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path") Cc: stable@vger.kernel.org Signed-off-by: Deepanshu Kartikey kartikey406@gmail.com --- fs/ext4/extents.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c index ca5499e9412b..903578d5f68d 100644 --- a/fs/ext4/extents.c +++ b/fs/ext4/extents.c @@ -1708,7 +1708,9 @@ static int ext4_ext_correct_indexes(handle_t *handle, struct inode *inode, struct ext4_extent *ex; __le32 border; int k, err = 0; - + if (!path || depth < 0 || depth > EXT4_MAX_EXTENT_DEPTH) { + return -EFSCORRUPTED; + } eh = path[depth].p_hdr; ex = path[depth].p_ext;
@@ -4200,6 +4202,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode, unsigned int allocated_clusters = 0; struct ext4_allocation_request ar; ext4_lblk_t cluster_offset; + struct ext4_extent_header *eh;
ext_debug(inode, "blocks %u/%u requested\n", map->m_lblk, map->m_len); trace_ext4_ext_map_blocks_enter(inode, map->m_lblk, map->m_len, flags); @@ -4212,7 +4215,12 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode, }
depth = ext_depth(inode); - + eh = path[depth].p_hdr; + if (!eh || le16_to_cpu(eh->eh_magic) != EXT4_EXT_MAGIC) { + EXT4_ERROR_INODE(inode, "invalid extent header after find_extent"); + err = -EFSCORRUPTED; + goto out; + } /* * consistent leaf must not be empty; * this situation is possible, though, _during_ tree modification;
Hello,
syzbot has tested the proposed patch but the reproducer is still triggering an issue: KASAN: slab-out-of-bounds Read in ext4_find_extent
EXT4-fs (loop4): stripe (1570) is not aligned with cluster size (16), stripe is disabled ================================================================== BUG: KASAN: slab-out-of-bounds in ext4_ext_binsearch fs/ext4/extents.c:841 [inline] BUG: KASAN: slab-out-of-bounds in ext4_find_extent+0xae6/0xcc0 fs/ext4/extents.c:956 Read of size 4 at addr ffff8880739bce18 by task syz.4.64/6825
CPU: 1 UID: 0 PID: 6825 Comm: syz.4.64 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025 Call Trace: <TASK> dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline] print_report+0xca/0x240 mm/kasan/report.c:482 kasan_report+0x118/0x150 mm/kasan/report.c:595 ext4_ext_binsearch fs/ext4/extents.c:841 [inline] ext4_find_extent+0xae6/0xcc0 fs/ext4/extents.c:956 ext4_ext_map_blocks+0x27c/0x3880 fs/ext4/extents.c:4211 ext4_map_query_blocks+0x13b/0x930 fs/ext4/inode.c:550 ext4_map_blocks+0x4b3/0x1740 fs/ext4/inode.c:773 _ext4_get_block+0x200/0x4c0 fs/ext4/inode.c:910 ext4_get_block_unwritten+0x2e/0x100 fs/ext4/inode.c:943 ext4_block_write_begin+0x990/0x1710 fs/ext4/inode.c:1198 ext4_write_begin+0xc04/0x19a0 fs/ext4/ext4_jbd2.h:-1 ext4_da_write_begin+0x445/0xda0 fs/ext4/inode.c:3129 generic_perform_write+0x2c2/0x900 mm/filemap.c:4175 ext4_buffered_write_iter+0xce/0x3a0 fs/ext4/file.c:299 ext4_file_write_iter+0x298/0x1bc0 fs/ext4/file.c:-1 new_sync_write fs/read_write.c:593 [inline] vfs_write+0x5c9/0xb30 fs/read_write.c:686 ksys_pwrite64 fs/read_write.c:793 [inline] __do_sys_pwrite64 fs/read_write.c:801 [inline] __se_sys_pwrite64 fs/read_write.c:798 [inline] __x64_sys_pwrite64+0x193/0x220 fs/read_write.c:798 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f105318e969 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 a8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007f1054026038 EFLAGS: 00000246 ORIG_RAX: 0000000000000012 RAX: ffffffffffffffda RBX: 00007f10533b5fa0 RCX: 00007f105318e969 RDX: 000000000000fdef RSI: 0000200000000140 RDI: 0000000000000004 RBP: 00007f1053210ab1 R08: 0000000000000000 R09: 0000000000000000 R10: 000000000000fecc R11: 0000000000000246 R12: 0000000000000000 R13: 0000000000000000 R14: 00007f10533b5fa0 R15: 00007ffeabbc1e38 </TASK>
Allocated by task 6341: kasan_save_stack mm/kasan/common.c:47 [inline] kasan_save_track+0x3e/0x80 mm/kasan/common.c:68 poison_kmalloc_redzone mm/kasan/common.c:388 [inline] __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:405 kasan_kmalloc include/linux/kasan.h:260 [inline] __do_kmalloc_node mm/slub.c:4376 [inline] __kmalloc_noprof+0x27a/0x4f0 mm/slub.c:4388 kmalloc_noprof include/linux/slab.h:909 [inline] kzalloc_noprof include/linux/slab.h:1039 [inline] __register_sysctl_table+0x72/0x1340 fs/proc/proc_sysctl.c:1379 __addrconf_sysctl_register+0x328/0x4c0 net/ipv6/addrconf.c:7321 addrconf_sysctl_register+0x168/0x1c0 net/ipv6/addrconf.c:7369 ipv6_add_dev+0xd46/0x1370 net/ipv6/addrconf.c:460 addrconf_notify+0x794/0x1010 net/ipv6/addrconf.c:3650 notifier_call_chain+0x1b6/0x3e0 kernel/notifier.c:85 call_netdevice_notifiers_extack net/core/dev.c:2267 [inline] call_netdevice_notifiers net/core/dev.c:2281 [inline] register_netdevice+0x1608/0x1ae0 net/core/dev.c:11244 register_netdev+0x40/0x60 net/core/dev.c:11322 sit_init_net+0x228/0x5c0 net/ipv6/sit.c:1867 ops_init+0x359/0x5c0 net/core/net_namespace.c:137 setup_net+0xfe/0x320 net/core/net_namespace.c:445 copy_net_ns+0x34e/0x4e0 net/core/net_namespace.c:580 create_new_namespaces+0x3f3/0x720 kernel/nsproxy.c:110 unshare_nsproxy_namespaces+0x11c/0x170 kernel/nsproxy.c:218 ksys_unshare+0x4c8/0x8c0 kernel/fork.c:3132 __do_sys_unshare kernel/fork.c:3203 [inline] __se_sys_unshare kernel/fork.c:3201 [inline] __x64_sys_unshare+0x38/0x50 kernel/fork.c:3201 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff8880739bc000 which belongs to the cache kmalloc-cg-4k of size 4096 The buggy address is located 1504 bytes to the right of allocated 2104-byte region [ffff8880739bc000, ffff8880739bc838)
The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x739b8 head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 memcg:ffff888027c52441 flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff) page_type: f5(slab) raw: 00fff00000000040 ffff88801a04b500 dead000000000122 0000000000000000 raw: 0000000000000000 0000000000040004 00000000f5000000 ffff888027c52441 head: 00fff00000000040 ffff88801a04b500 dead000000000122 0000000000000000 head: 0000000000000000 0000000000040004 00000000f5000000 ffff888027c52441 head: 00fff00000000003 ffffea0001ce6e01 00000000ffffffff 00000000ffffffff head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008 page dumped because: kasan: bad access detected page_owner tracks the page as allocated page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 6341, tgid 6341 (syz-executor), ts 124779228140, free_ts 121882435080 set_page_owner include/linux/page_owner.h:32 [inline] post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851 prep_new_page mm/page_alloc.c:1859 [inline] get_page_from_freelist+0x21d5/0x22b0 mm/page_alloc.c:3858 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416 alloc_slab_page mm/slub.c:2492 [inline] allocate_slab+0x8a/0x370 mm/slub.c:2660 new_slab mm/slub.c:2714 [inline] ___slab_alloc+0xbeb/0x1420 mm/slub.c:3901 __slab_alloc mm/slub.c:3992 [inline] __slab_alloc_node mm/slub.c:4067 [inline] slab_alloc_node mm/slub.c:4228 [inline] __kmalloc_cache_noprof+0x296/0x3d0 mm/slub.c:4402 kmalloc_noprof include/linux/slab.h:905 [inline] kzalloc_noprof include/linux/slab.h:1039 [inline] snmp6_alloc_dev net/ipv6/addrconf.c:362 [inline] ipv6_add_dev+0x6ca/0x1370 net/ipv6/addrconf.c:413 addrconf_notify+0x794/0x1010 net/ipv6/addrconf.c:3650 notifier_call_chain+0x1b6/0x3e0 kernel/notifier.c:85 call_netdevice_notifiers_extack net/core/dev.c:2267 [inline] call_netdevice_notifiers net/core/dev.c:2281 [inline] register_netdevice+0x1608/0x1ae0 net/core/dev.c:11244 register_netdev+0x40/0x60 net/core/dev.c:11322 sit_init_net+0x228/0x5c0 net/ipv6/sit.c:1867 ops_init+0x359/0x5c0 net/core/net_namespace.c:137 setup_net+0xfe/0x320 net/core/net_namespace.c:445 copy_net_ns+0x34e/0x4e0 net/core/net_namespace.c:580 page last free pid 6267 tgid 6267 stack trace: reset_page_owner include/linux/page_owner.h:25 [inline] free_pages_prepare mm/page_alloc.c:1395 [inline] __free_frozen_pages+0xbb1/0xd20 mm/page_alloc.c:2895 vfree+0x25a/0x400 mm/vmalloc.c:3434 kcov_put kernel/kcov.c:439 [inline] kcov_close+0x28/0x50 kernel/kcov.c:535 __fput+0x44c/0xa70 fs/file_table.c:468 task_work_run+0x1d1/0x260 kernel/task_work.c:227 exit_task_work include/linux/task_work.h:40 [inline] do_exit+0x6b5/0x2300 kernel/exit.c:961 do_group_exit+0x21c/0x2d0 kernel/exit.c:1102 get_signal+0x125e/0x1310 kernel/signal.c:3034 arch_do_signal_or_restart+0x9a/0x750 arch/x86/kernel/signal.c:337 exit_to_user_mode_loop+0x75/0x110 kernel/entry/common.c:40 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline] syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline] syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline] do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100 entry_SYSCALL_64_after_hwframe+0x77/0x7f
Memory state around the buggy address: ffff8880739bcd00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8880739bcd80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880739bce00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^ ffff8880739bce80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8880739bcf00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ==================================================================
Tested on:
commit: 22bdd6e6 Merge tag 'x86_apic_for_v6.18_rc1' of git://g.. git tree: upstream console output: https://syzkaller.appspot.com/x/log.txt?x=134485cd980000 kernel config: https://syzkaller.appspot.com/x/.config?x=6899ea2dd30a3ed1 dashboard link: https://syzkaller.appspot.com/bug?extid=9db318d6167044609878 compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8 patch: https://syzkaller.appspot.com/x/patch.diff?x=114e8260580000
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