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Message-ID: <Zg/iGQCDKa9bllyI@shell.armlinux.org.uk>
Date: Fri, 5 Apr 2024 12:35:53 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Mark Rutland <mark.rutland@....com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
syzbot <syzbot+186522670e6722692d86@...kaller.appspotmail.com>,
LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
bpf <bpf@...r.kernel.org>
Subject: Re: [syzbot] [mm?] BUG: unable to handle kernel paging request in
copy_from_kernel_nofault (2)
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@...ux-foundationorg> wrote:
> > >
> > > On Mon, 01 Apr 2024 22:19:25 -0700 syzbot <syzbot+186522670e6722692d86@...kaller.appspotmail.com> wrote:
> > >
> > > > Hello,
> > >
> > > Thanks. Cc: bpf@...r.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.
So I think BPF folk need to urgently state what checks are done on
the destination value for _any_ function that BPF can call which
writes to memory.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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