Re: Aarch64 EXT4FS inode checksum failures - seems to be weak memory ordering issues

Russell King - ARM Linux admin
2021-01-07T11:20:06Z

On Wed, Jan 06, 2021 at 10:32:23PM +0000, Russell King - ARM Linux admin wrote:
> On Wed, Jan 06, 2021 at 05:20:34PM +0000, Will Deacon wrote:
> > With that, I see the following after ten seconds or so:
> >
> > EXT4-fs error (device sda2): ext4_lookup:1707: inode #674497: comm md5sum: iget: checksum invalid
> >
> > Russell, Mark -- does this recipe explode reliably for you too?
>
> I've been working this evening on tracking down what change in the
> Kconfig file between your working 5.10 kernel binary you supplied me,
> and my failing 5.9 kernel.
>
> I've found that _enabling_ CONFIG_STACKPROTECTOR appears to mask the
> inode checksum failure problem, at least from a short test.) I'm going
> to re-enable CONFIG_STACKPROTECTOR and leave it running for longer.
>
> That is:
>
> CONFIG_STACKPROTECTOR=y
> CONFIG_STACKPROTECTOR_STRONG=y
>
> appears to mask the problem
>
> # CONFIG_STACKPROTECTOR is not set
>
> appears to unmask the problem.

We have finally got to the bottom of this - the "bug" is in the ext4
code:

static inline u32 ext4_chksum(struct ext4_sb_info *sbi, u32 crc,
const void *address, unsigned int length)
{
struct {
struct shash_desc shash;
char ctx[4];
} desc;

BUG_ON(crypto_shash_descsize(sbi->s_chksum_driver)!=sizeof(desc.ctx));

desc.shash.tfm = sbi->s_chksum_driver;
*(u32 *)desc.ctx = crc;

BUG_ON(crypto_shash_update(&desc.shash, address, length));

return *(u32 *)desc.ctx;
}

This isn't always inlined, despite the "inline" keyword. With GCC
4.9.4, this is compiled to the following code when the stack protector
is disabled:

0000000000000004 <ext4_chksum.isra.14.constprop.19>:
4: a9be7bfd stp x29, x30, [sp, #-32]! <------
8: 2a0103e3 mov w3, w1
c: aa0203e1 mov x1, x2
10: 910003fd mov x29, sp <------
14: f9000bf3 str x19, [sp, #16]
18: d10603ff sub sp, sp, #0x180 <------
1c: 9101fff3 add x19, sp, #0x7f
20: b9400002 ldr w2, [x0]
24: 9279e273 and x19, x19, #0xffffffffffffff80 <------
28: 7100105f cmp w2, #0x4
2c: 540001a1 b.ne 60 <ext4_chksum.isra.14.constprop.19+0x5c> // b.any
30: 2a0303e4 mov w4, w3
34: aa0003e3 mov x3, x0
38: b9008264 str w4, [x19, #128]
3c: aa1303e0 mov x0, x19
40: f9000263 str x3, [x19] <------
44: 94000000 bl 0 <crypto_shash_update>
44: R_AARCH64_CALL26 crypto_shash_update
48: 350000e0 cbnz w0, 64 <ext4_chksum.isra.14.constprop.19+0x60>
4c: 910003bf mov sp, x29 <======
50: b9408260 ldr w0, [x19, #128] <======
54: f9400bf3 ldr x19, [sp, #16]
58: a8c27bfd ldp x29, x30, [sp], #32
5c: d65f03c0 ret
60: d4210000 brk #0x800
64: 97ffffe7 bl 0 <ext4_chksum.isra.14.part.15>

Of the instructions that are highlighted with "<------" and "<======",
x29 is located at the bottom of the function's stack frame, excluding
local variables. x19 is "desc", which is calculated to be safely below
x29 and aligned to a 128 byte boundary.

The bug is pointed to by the two "<======" markers - the instruction
at 4c restores the stack pointer _above_ "desc" before then loading
desc.ctx.

If an interrupt occurs right between these two instructions, then
desc.ctx will be corrupted, leading to the checksum failing.

Comments on irc are long the lines of this being "an impressive
compiler bug".

We now need to find which gcc versions are affected, so we know what
minimum version to require for aarch64.

Arnd has been unable to find anything in gcc bugzilla to explain this;
he's tested gcc-5.5.0, which appears to produce correct code, and is
trying to bisect between 4.9.4 and 5.1.0 to locate where this was
fixed.

Peter Zijlstra suggested adding linux-toolchains@ and asking compiler
folks for feedback on this bug. I guess a pointer to whether this is
a known bug, and which bug may be useful.

I am very relieved to have found a positive reason for this bug, rather
than just moving forward on the compiler and have the bug vanish
without explanation, never knowing if it would rear its head in future
and corrupt my filesystems, e.g. never knowing if it became a
temporarily masked memory ordering bug.

--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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