BUG: memcmp(): Accessing invalid memory location

Wed Feb 6 23:42:21 AEDT 2019

On Wednesday, February 6, 2019 5:20:04 PM IST Michael Ellerman wrote:
> Chandan Rajendra <chandan at linux.ibm.com> writes:
> > On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:
> >> Michael Ellerman <mpe at ellerman.id.au> writes:
> >> 
> >> > Adding Simon who wrote the code.
> >> >
> >> > Chandan Rajendra <chandan at linux.ibm.com> writes:
> >> >> 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.
> >> >
> >> > Thanks for the analysis.
> >> >
> >> > This is just a bug, we can't read past the end of the source or dest.
> >> 
> >> How about this, works for me.
> >> 
> >> cheers
> >> 
> >> diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
> >> index 844d8e774492..2a302158cb53 100644
> >> --- a/arch/powerpc/lib/memcmp_64.S
> >> +++ b/arch/powerpc/lib/memcmp_64.S
> >> @@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
> >>  	beq	.Lzero
> >>  
> >>  .Lcmp_rest_lt8bytes:
> >> -	/* Here we have only less than 8 bytes to compare with. at least s1
> >> -	 * Address is aligned with 8 bytes.
> >> -	 * The next double words are load and shift right with appropriate
> >> -	 * bits.
> >> +	/*
> >> +	 * Here we have less than 8 bytes left to compare with. We mustn't read
> >> +	 * past the end of either source or dest.
> >>  	 */
> >> -	subfic  r6,r5,8
> >> -	slwi	r6,r6,3
> >> -	LD	rA,0,r3
> >> -	LD	rB,0,r4
> >> -	srd	rA,rA,r6
> >> -	srd	rB,rB,r6
> >> -	cmpld	cr0,rA,rB
> >> +
> >> +	/* If we have less than 4 bytes, just do byte at a time */
> >> +	cmpwi   cr1, r5, 4
> >> +	blt	cr1, .Lshort
> >> +
> >> +	/* Compare 4 bytes */
> >> +	LW	rA,0,r3
> >> +	LW	rB,0,r4
> >> +	cmpd	cr0,rA,rB
> >>  	bne	cr0,.LcmpAB_lightweight
> >> -	b	.Lzero
> >> +
> >> +	/* If we had exactly 4 bytes left, we're done now */
> >> +	beq	cr1, .Lzero
> >> +
> >> +	/* Otherwise do what ever's left a byte at a time */
> >> +	subi	r5, r5, 4
> >> +	addi	r3, r3, 4
> >> +	addi	r4, r4, 4
> >> +	b	.Lshort
> >>  
> >>  .Lnon_zero:
> >>  	mr	r3,rC
> >> 
> >> 
> >
> > With the above patch, Linux kernel does not end up in oops. Hence,
> >
> > Tested-by: Chandan Rajendra <chandan at linux.ibm.com>
> 
> Thanks.
> 
> How many times had you hit the original oops? ie. was it easily
> reproducible?

I could recreate the issue within 20 iterations of the test. For verifying
your patch, I let the test run for 500 iterations.

-- 
chandan