linux-kernel - Re: [PATCH 2/3] x86/entry: Fix entry/exit mismatch on failed fast 32-bit syscalls

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Message-Id: <AA835AD1-31C1-4C8A-AEFF-F0D1DD2C834C@amacapital.net>
Date:   Tue, 23 Feb 2021 07:35:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/entry: Fix entry/exit mismatch on failed fast 32-bit syscalls



> On Feb 23, 2021, at 3:29 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Feb 22, 2021 at 09:50:28PM -0800, Andy Lutomirski wrote:
>> On a 32-bit fast syscall that fails to read its arguments from user
>> memory, the kernel currently does syscall exit work but not
>> syscall exit work.  This would confuse audit and ptrace.
>> 
>> This is a minimal fix intended for ease of backporting.  A more
>> complete cleanup is coming.
>> 
>> Cc: stable@...r.kernel.org
>> Fixes: 0b085e68f407 ("x86/entry: Consolidate 32/64 bit syscall entry")
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>> arch/x86/entry/common.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>> index 0904f5676e4d..cf4dcf346ca8 100644
>> --- a/arch/x86/entry/common.c
>> +++ b/arch/x86/entry/common.c
>> @@ -128,7 +128,8 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
>>        regs->ax = -EFAULT;
>> 
>>        instrumentation_end();
>> -        syscall_exit_to_user_mode(regs);
>> +        local_irq_disable();
>> +        exit_to_user_mode();
>>        return false;
>>    }
> 
> I'm confused, twice. Once by your Changelog, and second by the actual
> patch. Shouldn't every return to userspace pass through
> exit_to_user_mode_prepare() ? We shouldn't ignore NEED_RESCHED or
> NOTIFY_RESUME, both of which can be set I think, even if the SYSCALL
> didn't actually do anything.


Aaaaahhhhhh!  There are too many of these functions. I’ll poke around. I’ll also try to figure out why I didn’t catch this in testing.