Hi upstream community, I was fuzzing a LTS version of Linux kernel 5.15.148 with my modified syzkaller and I found a bug named "KASAN: use-after-free in xfs_allocbt_init_key_from_rec". I tested the PoC on 5.15.148, 5.15.149 and 5.15.150 with sanitizer on and found sanitizer through a panic as "KASAN: use-after-free in xfs_allocbt_init_key_from_rec" on 5.15.148 and 5.15.150, but there was no panic and sanitizer error in 5.15.149. The syzkaller log, report, kernel config, PoC can be found here: https://drive.google.com/file/d/1w6VKKewt4VQzb9FzcGtkELJUOwd1wMcC/view?usp=sharing # Analysis (rough): Because that I cannot understand the report0 clearly in the zip file above, so I rerun the PoC on my vm (5.15.148) and I get another report named as the same but it looks much clearer than the report0. The new report can be found in: https://drive.google.com/file/d/1Vg_4Qwueow6VgjLrijnUB8QbZVx902sv/view?usp=sharing In this report, we can easily see that the memory allocation and free: Allocation: ``` [ 62.995194][ T6349] Allocated by task 6343: [ 62.995610][ T6349] kasan_save_stack+0x1b/0x40 [ 62.996044][ T6349] __kasan_slab_alloc+0x61/0x80 [ 62.996475][ T6349] kmem_cache_alloc+0x18e/0x6b0 [ 62.996918][ T6349] getname_flags+0xd2/0x5b0 [ 62.997335][ T6349] user_path_at_empty+0x2b/0x60 [ 62.997782][ T6349] vfs_statx+0x13c/0x370 [ 62.998193][ T6349] __do_sys_newlstat+0x91/0x110 [ 62.998634][ T6349] do_syscall_64+0x35/0xb0 [ 62.999033][ T6349] entry_SYSCALL_64_after_hwframe+0x61/0xcb ``` Free: ``` [ 62.999776][ T6349] Freed by task 6343: [ 63.000135][ T6349] kasan_save_stack+0x1b/0x40 [ 63.000555][ T6349] kasan_set_track+0x1c/0x30 [ 63.001053][ T6349] kasan_set_free_info+0x20/0x30 [ 63.001638][ T6349] __kasan_slab_free+0xe1/0x110 [ 63.002206][ T6349] kmem_cache_free+0x82/0x5b0 [ 63.002742][ T6349] putname+0xfe/0x140 [ 63.003103][ T6349] user_path_at_empty+0x4d/0x60 [ 63.003551][ T6349] vfs_statx+0x13c/0x370 [ 63.003943][ T6349] __do_sys_newlstat+0x91/0x110 [ 63.004378][ T6349] do_syscall_64+0x35/0xb0 [ 63.004841][ T6349] entry_SYSCALL_64_after_hwframe+0x61/0xcb ``` So this is a use-after-free bug: allocated by `kmem_cache_alloc` and freed by `kmem_cache_free`. And according to the report, the UAF occurs in `xfs_allocbt_init_key_from_rec`, `key->alloc.ar_startblock = rec->alloc.ar_startblock;` which indicates that maybe `rec->alloc.ar_startblock` was freed before. # Step to reproduce: 1. download the zip file 2. unzip it 3. compile the kernel (5.15.148, 5.15.150) with kernel_config 4. start the kernel with qemu vm 5. scp repro.c to the vm 6. compile the repro.c and run it: gcc repro.c -o exp && ./exp 7. you will see the KASAN error # Note: I didn't find any related reports on the internet, which indicates that it may be a 0day. Hope the upstream can help check and fix it. And I'll be happy to provide more information if needed. Best, Tong
