Re: [PATCH RESEND] exec: don't force_sigsegv processes with a pending fatal signal

[Dmitry Safonov]

2018-08-02 20:53 GMT+01:00 Dmitry Safonov <0x7f454c46@xxxxxxxxx>:
> Hi Ivan,
>
> 2018-07-31 1:56 GMT+01:00 Ivan Delalande <colona@xxxxxxxxxx>:
>> We were seeing unexplained segfaults in coreutils processes and other
>> basic utilities that we tracked down to binfmt_elf failing to load
>> segments for ld.so. Digging further, the actual problem seems to occur
>> when a process gets sigkilled while it is still being loaded by the
>> kernel. In our case when _do_page_fault goes for a retry it will return
>> early as it first checks for fatal_signal_pending(), so load_elf_interp
>> also returns with error and as a result search_binary_handler will
>> force_sigsegv() which is pretty confusing as nothing actually failed
>> here.
>>
>> Fixes: 19d860a140be ("handle suicide on late failure exits in execve() in search_binary_handler()")
>> Reference: https://lkml.org/lkml/2013/2/14/5
>> Signed-off-by: Ivan Delalande <colona@xxxxxxxxxx>
>
> +Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> +Cc: Andy Lutomirski <luto@xxxxxxxxxx>

Also worth to add to commit message an example of previously
user-visible message in dmesg:

[ 1545.889604] potentially unexpected fatal signal 11.
[ 1545.889614] CPU: 2 PID: 7462 Comm: grep Tainted: P           O   3.18.28 #1
[ 1545.889617] Hardware name: Celestica D4040/D4040, BIOS 5.6.5 08/18/2016
[ 1545.889621] task: ffff880011282280 ti: ffff880100938000 task.ti:
ffff880100938000
[ 1545.889624] RIP: 0023:[<00000000f760eb70>]  [<00000000f760eb70>] 0xf760eb70
[ 1545.889641] RSP: 002b:00000000fffa3454  EFLAGS: 00000296
[ 1545.889644] RAX: fffffffffffffff2 RBX: 00000000f7a5c3e8 RCX: 00000000f7a5c718
[ 1545.889647] RDX: 00000000f7a5b230 RSI: 00000000f7a5c718 RDI: 00000000f757c000
[ 1545.889650] RBP: 00000000f7a5c3e8 R08: 0000000000000000 R09: 0000000000000000
[ 1545.889653] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 1545.889656] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[ 1545.889659] FS:  0000000000000000(0000) GS:ffff88017fb00000(0000)
knlGS:0000000000000000
[ 1545.889662] CS:  0010 DS: 002b ES: 002b CR0: 000000008005003b
[ 1545.889665] CR2: 00000000f77d7838 CR3: 000000010bb90000 CR4: 00000000001007e0

(which now will be suppressed if there was a fatal signal)

>
>> ---
>>  fs/exec.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index bdd0eacefdf5..6e8007edbb2d 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1656,7 +1656,8 @@ int search_binary_handler(struct linux_binprm *bprm)
>>                 if (retval < 0 && !bprm->mm) {
>>                         /* we got to flush_old_exec() and failed after it */
>>                         read_unlock(&binfmt_lock);
>> -                       force_sigsegv(SIGSEGV, current);
>> +                       if (!fatal_signal_pending(current))
>> +                               force_sigsegv(SIGSEGV, current);
>
> I would suggest to add something like:
> : if (print_fatal_signals)
> :         pr_info("load_binary() failed: %d\n", retval);
>
> It was interesting to catch that it actually segfaults during loading,
> probably will save someone a couple of minutes too ;-)

Not sure if it's easy to trigger, but it might require a ratelimit too..

>
>>                         return retval;
>>                 }
>>                 if (retval != -ENOEXEC || !bprm->file) {

-- 
             Dmitry