Re: [syzbot] [mm?] BUG: unable to handle kernel paging request in copy_from_kernel_nofault (2)

[Russell King (Oracle)]

On Tue, Apr 09, 2024 at 07:45:54AM +0000, Puranjay Mohan wrote:
> "Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx> writes:
> 
> > On Fri, Apr 05, 2024 at 10:50:30AM -0700, Andrii Nakryiko wrote:
> >> On Fri, Apr 5, 2024 at 9:30 AM Alexei Starovoitov
> >> <alexei.starovoitov@xxxxxxxxx> wrote:
> >> >
> >> > On Fri, Apr 5, 2024 at 4:36 AM Russell King (Oracle)
> >> > <linux@xxxxxxxxxxxxxxx> wrote:
> >> > >
> >> > > On Fri, Apr 05, 2024 at 12:02:36PM +0100, Mark Rutland wrote:
> >> > > > On Thu, Apr 04, 2024 at 03:57:04PM -0700, Alexei Starovoitov wrote:
> >> > > > > On Wed, Apr 3, 2024 at 6:56 PM Andrew Morton <akpm@linux-foundationorg> wrote:
> >> > > > > >
> >> > > > > > On Mon, 01 Apr 2024 22:19:25 -0700 syzbot <syzbot+186522670e6722692d86@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >> > > > > >
> >> > > > > > > Hello,
> >> > > > > >
> >> > > > > > Thanks.  Cc: bpf@xxxxxxxxxxxxxxx
> >> > > > >
> >> > > > > I suspect the issue is not on bpf side.
> >> > > > > Looks like the bug is somewhere in arm32 bits.
> >> > > > > copy_from_kernel_nofault() is called from lots of places.
> >> > > > > bpf is just one user that is easy for syzbot to fuzz.
> >> > > > > Interestingly arm defines copy_from_kernel_nofault_allowed()
> >> > > > > that should have filtered out user addresses.
> >> > > > > In this case ffffffe9 is probably a kernel address?
> >> > > >
> >> > > > It's at the end of the kernel range, and it's ERR_PTR(-EINVAL).
> >> > > >
> >> > > > 0xffffffe9 is -0x16, which is -22, which is -EINVAL.
> >> > > >
> >> > > > > But the kernel is doing a write?
> >> > > > > Which makes no sense, since copy_from_kernel_nofault is probe reading.
> >> > > >
> >> > > > It makes perfect sense; the read from 'src' happened, then the kernel tries to
> >> > > > write the result to 'dst', and that aligns with the disassembly in the report
> >> > > > below, which I beleive is:
> >> > > >
> >> > > >      8: e4942000        ldr     r2, [r4], #0  <-- Read of 'src', fault fixup is elsewhere
> >> > > >      c: e3530000        cmp     r3, #0
> >> > > >   * 10: e5852000        str     r2, [r5]      <-- Write to 'dst'
> >> > > >
> >> > > > As above, it looks like 'dst' is ERR_PTR(-EINVAL).
> >> > > >
> >> > > > Are you certain that BPF is passing a sane value for 'dst'? Where does that
> >> > > > come from in the first place?
> >> > >
> >> > > It looks to me like it gets passed in from the BPF program, and the
> >> > > "type" for the argument is set to ARG_PTR_TO_UNINIT_MEM. What that
> >> > > means for validation purposes, I've no idea, I'm not a BPF hacker.
> >> > >
> >> > > Obviously, if BPF is allowing copy_from_kernel_nofault() to be passed
> >> > > an arbitary destination address, that would be a huge security hole.
> >> >
> >> > If that's the case that's indeed a giant security hole,
> >> > but I doubt it. We would be crashing other archs as well.
> >> > I cannot really tell whether arm32 JIT is on.
> >> > If it is, it's likely a bug there.
> >> > Puranjay,
> >> > could you please take a look.
> >> >
> >> 
> >> I dumped the BPF program that repro.c is loading, it works on x86-64
> >> and there is nothing special there. We are probe-reading 5 bytes from
> >> somewhere into the stack. Everything is unaligned here, but stays
> >> within a well-defined memory slot.
> >> 
> >> Note the r3 = (s8)r1, that's a new-ish thing, maybe bug is somewhere
> >> there (but then it would be JIT, not verifier itself)
> >> 
> >>    0: (7a) *(u64 *)(r10 -8) = 896542069
> >>    1: (bf) r1 = r10
> >>    2: (07) r1 += -7
> >>    3: (b7) r2 = 5
> >>    4: (bf) r3 = (s8)r1
> >>    5: (85) call bpf_probe_read_kernel#-72390
> >
> 
> I have started looking into this, the issue only reproduces when the JIT
> is enabled. With the interpreter, it works fine.
> 
> I used GDB to dump the JITed BPF program:
> 
>    0xbf00012c:  push    {r4, r5, r6, r7, r8, r9, r11, lr}
>    0xbf000130:  mov     r11, sp
>    0xbf000134:  mov     r3, #0
>    0xbf000138:  sub     r2, sp, #80     @ 0x50
>    0xbf00013c:  sub     sp, sp, #88     @ 0x58
>    0xbf000140:  strd    r2, [r11, #-64] @ 0xffffffc0
>    0xbf000144:  mov     r2, #0
>    0xbf000148:  strd    r2, [r11, #-72] @ 0xffffffb8
>    0xbf00014c:  mov     r2, r0
>    0xbf000150:  movw    r8, #9589       @ 0x2575
>    0xbf000154:  movt    r8, #13680      @ 0x3570
>    0xbf000158:  mov     r9, #0
>    0xbf00015c:  ldr     r6, [r11, #-64] @ 0xffffffc0
>    0xbf000160:  str     r8, [r6, #-8]
>    0xbf000164:  str     r9, [r6, #-4]
>    0xbf000168:  ldrd    r2, [r11, #-64] @ 0xffffffc0
>    0xbf00016c:  movw    r8, #65529      @ 0xfff9
>    0xbf000170:  movt    r8, #65535      @ 0xffff
>    0xbf000174:  movw    r9, #65535      @ 0xffff
>    0xbf000178:  movt    r9, #65535      @ 0xffff
>    0xbf00017c:  adds    r2, r2, r8
>    0xbf000180:  adc     r3, r3, r9
>    0xbf000184:  mov     r6, #5
>    0xbf000188:  mov     r7, #0
>    0xbf00018c:  strd    r6, [r11, #-8]
>    0xbf000190:  ldrd    r6, [r11, #-16]

Up to this point, it looks correct. r2/r3 contain the stack pointer
which corresponds to the instruction at "2:"

>    0xbf000194:  lsl     r2, r2, #24
>    0xbf000198:  asr     r2, r2, #24
>    0xbf00019c:  str     r2, [r11, #-16]

This then narrows the 64-bit pointer down to just 8!!! bits, but this
is what the instruction at "4:" is asking for. However, it looks like
it's happening to BPF's "r1" rather than "r3" and this is probably
where the problem lies.

I haven't got time to analyse this further this morning - I'm only
around sporadically today. I'll try to look deeper at this later on.

-- 
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