BUG: memcmp(): Accessing invalid memory location

[Chandan Rajendra]

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.
  
-- 
chandan