Re: [PATCH bpf v3 2/3] x86/mm: Disallow vsyscall page read for copy_from_kernel_nofault()

[Alexei Starovoitov]

On Fri, Feb 2, 2024 at 11:03 AM Sohil Mehta <sohil.mehta@xxxxxxxxx> wrote:
>
> On 2/2/2024 2:39 AM, Hou Tao wrote:
> > From: Hou Tao <houtao1@xxxxxxxxxx>
> >
> > When trying to use copy_from_kernel_nofault() to read vsyscall page
> > through a bpf program, the following oops was reported:
> >
> >   BUG: unable to handle page fault for address: ffffffffff600000
> >   #PF: supervisor read access in kernel mode
> >   #PF: error_code(0x0000) - not-present page
> >   PGD 3231067 P4D 3231067 PUD 3233067 PMD 3235067 PTE 0
> >   Oops: 0000 [#1] PREEMPT SMP PTI
> >   CPU: 1 PID: 20390 Comm: test_progs ...... 6.7.0+ #58
> >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
> >   RIP: 0010:copy_from_kernel_nofault+0x6f/0x110
> >   ......
> >   Call Trace:
> >    <TASK>
> >    ? copy_from_kernel_nofault+0x6f/0x110
> >    bpf_probe_read_kernel+0x1d/0x50
> >    bpf_prog_2061065e56845f08_do_probe_read+0x51/0x8d
> >    trace_call_bpf+0xc5/0x1c0
> >    perf_call_bpf_enter.isra.0+0x69/0xb0
> >    perf_syscall_enter+0x13e/0x200
> >    syscall_trace_enter+0x188/0x1c0
> >    do_syscall_64+0xb5/0xe0
> >    entry_SYSCALL_64_after_hwframe+0x6e/0x76
> >    </TASK>
> >   ......
> >   ---[ end trace 0000000000000000 ]---
> >
> > The oops is triggered when:
> >
> > 1) A bpf program uses bpf_probe_read_kernel() to read from the vsyscall
> > page and invokes copy_from_kernel_nofault() which in turn calls
> > __get_user_asm().
> >
> > 2) Because the vsyscall page address is not readable from kernel space,
> > a page fault exception is triggered accordingly.
> >
> > 3) handle_page_fault() considers the vsyscall page address as a user
> > space address instead of a kernel space address. This results in the
> > fix-up setup by bpf not being applied and a page_fault_oops() is invoked
> > due to SMAP.
> >
> > Considering handle_page_fault() has already considered the vsyscall page
> > address as a userspace address, fix the problem by disallowing vsyscall
> > page read for copy_from_kernel_nofault().
> >
> > Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Thomas,

could you please Ack the patch if you're still ok with it,
so we can take through the bpf tree to Linus soon ?

Not only syzbot, but real users are hitting this bug.

Thanks!

> > Reported-by: syzbot+72aa0161922eba61b50e@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@xxxxxxxxxxxxxx
> > Reported-by: xingwei lee <xrivendell7@xxxxxxxxx>
> > Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@xxxxxxxxxxxxxx
> > Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> > ---
> >  arch/x86/mm/maccess.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
>
> Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
>