Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Fri, Jan 03, 2025 at 03:25:01PM +0800, cheung wall wrote: >> I am writing to report a potential vulnerability identified in the >> Linux Kernel version 6.13.0-rc2. This issue was discovered using our >> custom vulnerability discovery tool. > > Your tool would be more useful if you told us what it was doing. > I suspect it's writing a very small value into the min_bytes pseudo-file. > Since that's something only root can do, this isn't a vulnerability. > This is a very annoying conversation to keep having with people who > write their own custom "vulnerability discovery tools". > > That said, we could do better here. Stefan, you wrote this code. > Thanks for the analysis Matthew. I'll have a look. Is there a testcase? >> RIP: 0010:div64_u64 include/linux/math64.h:69 [inline] >> RIP: 0010:bdi_ratio_from_pages mm/page-writeback.c:695 [inline] >> RIP: 0010:bdi_set_min_bytes+0x9f/0x1d0 mm/page-writeback.c:799 >> Code: ff 48 39 d8 0f 82 3b 01 00 00 e8 ac fd e7 ff 48 69 db 40 42 0f >> 00 48 8d 74 24 40 48 8d 7c 24 20 e8 c6 f1 ff ff 31 d2 48 89 d8 <48> f7 >> 74 24 40 48 89 c3 3d 40 42 0f 00 0f 87 08 01 00 00 e8 79 fd >> RSP: 0018:ffff88810a5f7b60 EFLAGS: 00010246 >> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff9c9ef057 >> RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff88810a5f7ab8 >> RBP: 1ffff110214bef6c R08: 0000000000000000 R09: fffffbfff4081c7b >> R10: ffffffffa040e3df R11: 0000000000032001 R12: ffff888105c65000 >> R13: dffffc0000000000 R14: ffff888105c65000 R15: ffff888105c65800 >> FS: 00007fdfc7c37580(0000) GS:ffff88811b280000(0000) knlGS:0000000000000000 >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> CR2: 000055adcdc786c8 CR3: 0000000104128000 CR4: 0000000000350ef0 >> Call Trace: >> <TASK> >> min_bytes_store+0xba/0x120 mm/backing-dev.c:385 >> dev_attr_store+0x58/0x80 drivers/base/core.c:2439 >> sysfs_kf_write+0x136/0x1a0 fs/sysfs/file.c:139 >> kernfs_fop_write_iter+0x323/0x530 fs/kernfs/file.c:334 >> new_sync_write fs/read_write.c:586 [inline] >> vfs_write+0x51e/0xc80 fs/read_write.c:679 >> ksys_write+0x110/0x200 fs/read_write.c:731 >> do_syscall_x64 arch/x86/entry/common.c:52 [inline] >> do_syscall_64+0xa6/0x1a0 arch/x86/entry/common.c:83 >> entry_SYSCALL_64_after_hwframe+0x77/0x7f >> RIP: 0033:0x7fdfc7b4d513 >> Code: 8b 15 81 29 0e 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f >> 1f 00 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 01 00 00 00 0f 05 <48> 3d >> 00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 48 89 54 24 18 >> RSP: 002b:00007ffe7796ae28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 >> RAX: ffffffffffffffda RBX: 000055adcdc766c0 RCX: 00007fdfc7b4d513 >> RDX: 0000000000000002 RSI: 000055adcdc766c0 RDI: 0000000000000001 >> RBP: 0000000000000002 R08: 000055adcdc766c0 R09: 00007fdfc7c30be0 >> R10: 0000000000000070 R11: 0000000000000246 R12: 0000000000000001 >> R13: 0000000000000002 R14: 7fffffffffffffff R15: 0000000000000000 >> >> ------------[ cut here end]------------ >> >> Root Cause: >> >> The crash is caused by a division by zero error within the Linux >> kernel's page-writeback subsystem. Specifically, the bdi_set_min_bytes >> function attempts to calculate a ratio using bdi_ratio_from_pages, >> which internally calls div64_u64. During this calculation, a >> denominator value unexpectedly becomes zero, likely due to improper >> handling or validation of input data provided through the sysfs >> interface during the min_bytes_store operation. This erroneous zero >> value leads to a divide error exception when the kernel tries to >> perform the division. The issue occurs while processing a sysfs write >> operation (min_bytes_store), suggesting that invalid or uninitialized >> data supplied through sysfs triggers the faulty calculation, >> ultimately causing the kernel to crash. >> >> Thank you for your time and attention. >> >> Best regards >> >> Wall
