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

[Puranjay Mohan <puranjay12@gmail.com>]

"Russell King (Oracle)" <linux@armlinux.org.uk> 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@gmail.com> wrote:
>> >
>> > On Fri, Apr 5, 2024 at 4:36 AM Russell King (Oracle)
>> > <linux@armlinux.org.uk> 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@syzkaller.appspotmail.com> wrote:
>> > > > > >
>> > > > > > > Hello,
>> > > > > >
>> > > > > > Thanks.  Cc: bpf@vger.kernel.org
>> > > > >
>> > > > > 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]
   0xbf000194:  lsl     r2, r2, #24
   0xbf000198:  asr     r2, r2, #24
   0xbf00019c:  str     r2, [r11, #-16]
   0xbf0001a0:  asr     r7, r6, #31
   0xbf0001a4:  mov     r1, r3
   0xbf0001a8:  mov     r0, r2
   0xbf0001ac:  ldrd    r2, [r11, #-8]
   0xbf0001b0:  ldrd    r8, [r11, #-32] @ 0xffffffe0
   0xbf0001b4:  push    {r8, r9}
   0xbf0001b8:  ldrd    r8, [r11, #-24] @ 0xffffffe8
   0xbf0001bc:  push    {r8, r9}
   0xbf0001c0:  ldrd    r8, [r11, #-16]
   0xbf0001c4:  push    {r8, r9}
   0xbf0001c8:  movw    r6, #40536      @ 0x9e58
   0xbf0001cc:  movt    r6, #49223      @ 0xc047
   0xbf0001d0:  blx     r6
   0xbf0001d4:  add     sp, sp, #24
   0xbf0001d8:  mov     r0, #0
   0xbf0001dc:  mov     r1, #0
   0xbf0001e0:  mov     sp, r11
   0xbf0001e4:  pop     {r4, r5, r6, r7, r8, r9, r11, pc}

Thanks,
Puranjay