linux-kernel - Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put

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Date:   Sun, 6 Oct 2019 19:30:16 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()

On 10/6/19 6:17 PM, Linus Torvalds wrote:
> On Sun, Oct 6, 2019 at 5:04 PM Guenter Roeck <linux@...ck-us.net> wrote:
[ ... ]
> And yes, I'll fix that name copy loop in filldir to align the
> destination first, *but* if I'm right, it means that something like
> this should also likely cause issues:
> 
>    #define _GNU_SOURCE
>    #include <unistd.h>
>    #include <sys/mman.h>
> 
>    int main(int argc, char **argv)
>    {
>          void *mymap;
>          uid_t *bad_ptr = (void *) 0x01;
> 
>          /* Create unpopulated memory area */
>          mymap = mmap(NULL, 16384, PROT_READ | PROT_WRITE, MAP_PRIVATE
> | MAP_ANONYMOUS, -1, 0);
> 
>          /* Unaligned uidpointer in that memory area */
>          bad_ptr = mymap+1;
> 
>          /* Make the kernel do put_user() on it */
>          return getresuid(bad_ptr, bad_ptr+1, bad_ptr+2);
>    }
> 
> because that simple user mode program should cause that same "page
> fault on unaligned put_user()" behavior as far as I can tell.
> 
> Mind humoring me and trying that on your alpha machine (or emulator,
> or whatever)?
> 

Here you are. This is with v5.4-rc2 and your previous patch applied
on top.

/ # ./mmtest
Unable to handle kernel paging request at virtual address 0000000000000004
mmtest(75): Oops -1
pc = [<0000000000000004>]  ra = [<fffffc0000311584>]  ps = 0000    Not tainted
pc is at 0x4
ra is at entSys+0xa4/0xc0
v0 = fffffffffffffff2  t0 = 0000000000000000  t1 = 0000000000000000
t2 = 0000000000000000  t3 = 0000000000000000  t4 = 0000000000000000
t5 = 000000000000fffe  t6 = 0000000000000000  t7 = fffffc0007edc000
s0 = 0000000000000000  s1 = 00000001200006f0  s2 = 00000001200df19f
s3 = 00000001200ea0b9  s4 = 0000000120114630  s5 = 00000001201145d8
s6 = 000000011f955c50
a0 = 000002000002c001  a1 = 000002000002c005  a2 = 000002000002c009
a3 = 0000000000000000  a4 = ffffffffffffffff  a5 = 0000000000000000
t8 = 0000000000000000  t9 = fffffc0000000000  t10= 0000000000000000
t11= 000000011f955788  pv = fffffc0000349450  at = 00000000f8db54d3
gp = fffffc0000f2a160  sp = 00000000ab237c72
Disabling lock debugging due to kernel taint
Trace:

Code:
  00000000
  00063301
  000007b6
  00001111
  00003f8d

Segmentation fault

Guenter