Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

[Ben Dooks]

On 11/03/2021 06:52, Dmitry Vyukov wrote:
> On Thu, Mar 11, 2021 at 7:50 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Thu, Mar 11, 2021 at 7:40 AM Alex Ghiti <alex@xxxxxxxx> wrote:
> > > Hi Ben,
> > >
> > > Le 3/10/21 à 5:24 PM, Ben Dooks a écrit :
> > > > On 10/03/2021 17:16, Dmitry Vyukov wrote:
> > > > > On Wed, Mar 10, 2021 at 5:46 PM syzbot
> > > > > <syzbot+e74b94fe601ab9552d69@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > Hello,
> > > > > >
> > > > > > syzbot found the following issue on:
> > > > > >
> > > > > > HEAD commit:    0d7588ab riscv: process: Fix no prototype for
> > > > > > arch_dup_tas..
> > > > > > git tree:
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1212c6e6d00000
> > > > > > kernel config:
> > > > > > https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
> > > > > > dashboard link:
> > > > > > https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
> > > > > > userspace arch: riscv64
> > > > > >
> > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > >
> > > > > > IMPORTANT: if you fix the issue, please add the following tag to the
> > > > > > commit:
> > > > > > Reported-by: syzbot+e74b94fe601ab9552d69@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > >
> > > > > +riscv maintainers
> > > > >
> > > > > This is riscv64-specific.
> > > > > I've seen similar crashes in put_user in other places. It looks like
> > > > > put_user crashes in the user address is not mapped/protected (?).
> > > >
> > > > The unmapped case should have been handled.
> > > >
> > > > I think this issue is that the check for user-mode access added. From
> > > > what I read the code may be wrong in
> > > >
> > > > +    if (!user_mode(regs) && addr < TASK_SIZE &&
> > > > +            unlikely(!(regs->status & SR_SUM)))
> > > > +        die_kernel_fault("access to user memory without uaccess routines",
> > > > +                addr, regs);
> > > >
> > > > I think the SR_SUM check might be wrong, as I read the standard the
> > > > SR_SUM should be set to disable user-space access. So the check
> > > > should be unlikely(regs->status & SR_SUM) to say access without
> > > > having disabled the protection.
> > >
> > > The check that is done seems correct to me: "The SUM (permit Supervisor
> > > User Memory access) bit modifies the privilege with which S-mode loads
> > > and stores access virtual memory.  *When SUM=0, S-mode memory accesses
> > > to pages that are accessible by U-mode (U=1 in Figure 4.15) will fault*.
> > >    When SUM=1, these accesses are permitted.SUM  has  no  effect  when
> > > page-based  virtual  memory  is  not  in  effect".
> > >
> > > I will try to reproduce the problem locally.
> >
> > Weird. It crashes with this all the time:
> > https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
> >
> > Even on trivial programs that almost don't do anything.
> > Maybe it's qemu bug? Do registers look sane in the dump? That SR_SUM, etc.
> >
> >
> > 00:13:27 executing program 1:
> > openat$drirender128(0xffffffffffffff9c,
> > &(0x7f0000000040)='/dev/dri/renderD128\x00', 0x0, 0x0)
> >
> > [  812.318182][ T4833] Unable to handle kernel access to user memory
> > without uaccess routines at virtual address 00000000250b60d0
> > [  812.322304][ T4833] Oops [#1]
> > [  812.323196][ T4833] Modules linked in:
> > [  812.324110][ T4833] CPU: 1 PID: 4833 Comm: syz-executor.1 Not
> > tainted 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> > [  812.325862][ T4833] Hardware name: riscv-virtio,qemu (DT)
> > [  812.327561][ T4833] epc : schedule_tail+0x72/0xb2
> > [  812.328640][ T4833]  ra : schedule_tail+0x70/0xb2
> > [  812.330088][ T4833] epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp
> > : ffffffe0238bbec0
> > [  812.331312][ T4833]  gp : ffffffe005d25378 tp : ffffffe00a275b00 t0
> > : 0000000000000000
> > [  812.333014][ T4833]  t1 : 0000000000000001 t2 : 00000000000f4240 s0
> > : ffffffe0238bbee0
> > [  812.334137][ T4833]  s1 : 00000000250b60d0 a0 : 0000000000000036 a1
> > : 0000000000000003
> > [  812.336063][ T4833]  a2 : 1ffffffc0cfa8b00 a3 : ffffffe0000c80cc a4
> > : 7f467e72c6adf800
> > [  812.337398][ T4833]  a5 : 0000000000000000 a6 : 0000000000f00000 a7
> > : ffffffe0000f8c84
> > [  812.339287][ T4833]  s2 : 0000000000040000 s3 : ffffffe0077a96c0 s4
> > : ffffffe020e67fe0
> > [  812.340658][ T4833]  s5 : 0000000000004020 s6 : ffffffe0077a9b58 s7
> > : ffffffe067d74850
> > [  812.342492][ T4833]  s8 : ffffffe067d73e18 s9 : 0000000000000000
> > s10: ffffffe00bd72280
> > [  812.343668][ T4833]  s11: 000000bd067bf638 t3 : 7f467e72c6adf800 t4
> > : ffffffc403ee7fb2
> > [  812.345510][ T4833]  t5 : ffffffc403ee7fba t6 : 0000000000040000
> > [  812.347004][ T4833] status: 0000000000000120 badaddr:
> > 00000000250b60d0 cause: 000000000000000f
> > [  812.348091][ T4833] Call Trace:
> > [  812.349291][ T4833] [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2
> > [  812.350796][ T4833] [<ffffffe000005570>] ret_from_exception+0x0/0x14
> > [  812.352799][ T4833] Dumping ftrace buffer:
> > [  812.354328][ T4833]    (ftrace buffer empty)
> > [  812.428145][ T4833] ---[ end trace 94b077e4d677ee73 ]---
> >
> >
> > 00:10:42 executing program 1:
> > bpf$ENABLE_STATS(0x20, 0x0, 0x0)
> > bpf$ENABLE_STATS(0x20, 0x0, 0x0)
> >
> > [  646.536862][ T5163] loop0: detected capacity change from 0 to 1
> > [  646.566730][ T5165] Unable to handle kernel access to user memory
> > without uaccess routines at virtual address 00000000032f80d0
> > [  646.586024][ T5165] Oops [#1]
> > [  646.586640][ T5165] Modules linked in:
> > [  646.587350][ T5165] CPU: 1 PID: 5165 Comm: syz-executor.1 Not
> > tainted 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> > [  646.588209][ T5165] Hardware name: riscv-virtio,qemu (DT)
> > [  646.589019][ T5165] epc : schedule_tail+0x72/0xb2
> > [  646.589811][ T5165]  ra : schedule_tail+0x70/0xb2
> > [  646.590435][ T5165] epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp
> > : ffffffe008013ec0
> > [  646.591142][ T5165]  gp : ffffffe005d25378 tp : ffffffe007634440 t0
> > : 0000000000000000
> > [  646.591836][ T5165]  t1 : 0000000000000001 t2 : 0000000000000008 s0
> > : ffffffe008013ee0
> > [  646.592509][ T5165]  s1 : 00000000032f80d0 a0 : 0000000000000004 a1
> > : 0000000000000003
> > [  646.593188][ T5165]  a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4
> > : 8d229faaffda9500
> > [  646.593878][ T5165]  a5 : 0000000000000000 a6 : 0000000000f00000 a7
> > : ffffffe000082eba
> > [  646.594552][ T5165]  s2 : 0000000000040000 s3 : ffffffe00c82c440 s4
> > : ffffffe00e61ffe0
> > [  646.595253][ T5165]  s5 : 0000000000004000 s6 : ffffffe067d57e00 s7
> > : ffffffe067d57850
> > [  646.595938][ T5165]  s8 : ffffffe067d56e18 s9 : ffffffe067d57e00
> > s10: ffffffe00c82c878
> > [  646.596627][ T5165]  s11: 000000967ba7a1cc t3 : 8d229faaffda9500 t4
> > : ffffffc4011bc79b
> > [  646.597319][ T5165]  t5 : ffffffc4011bc79d t6 : ffffffe008de3ce8
> > [  646.597909][ T5165] status: 0000000000000120 badaddr:
> > 00000000032f80d0 cause: 000000000000000f
> > [  646.598682][ T5165] Call Trace:
> > [  646.599294][ T5165] [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2
> > [  646.600115][ T5165] [<ffffffe000005570>] ret_from_exception+0x0/0x14
> > [  646.601333][ T5165] Dumping ftrace buffer:
> > [  646.602322][ T5165]    (ftrace buffer empty)
> > [  646.663691][ T5165] ---[ end trace e7b7847ce74cdfca ]---
>
> Is it reasonable that schedule_tail is called from ret_from_exception?
> Maybe the issue is in ret_from_exception? I see it does something with
> registers.

I'd not noticed this with an earlier kernel (5.10 and the user-fault
check patches) but this may be an qemu issue?


--
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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