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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 2/3] x86/entry: Fix entry/exit mismatch on failed fast 32-bit syscalls
Date:   Tue, 23 Feb 2021 07:35:47 -0800
Message-Id: <AA835AD1-31C1-4C8A-AEFF-F0D1DD2C834C@amacapital.net>
References: <YDTm8Q/cvBcfzhPn@hirez.programming.kicks-ass.net>
Cc:     Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org
In-Reply-To: <YDTm8Q/cvBcfzhPn@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Precedence: bulk


> On Feb 23, 2021, at 3:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> =EF=BB=BFOn 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.
>>=20
>> This is a minimal fix intended for ease of backporting.  A more
>> complete cleanup is coming.
>>=20
>> Cc: stable@vger.kernel.org
>> Fixes: 0b085e68f407 ("x86/entry: Consolidate 32/64 bit syscall entry")
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>> arch/x86/entry/common.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>=20
>> 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_re=
gs *regs)
>>        regs->ax =3D -EFAULT;
>>=20
>>        instrumentation_end();
>> -        syscall_exit_to_user_mode(regs);
>> +        local_irq_disable();
>> +        exit_to_user_mode();
>>        return false;
>>    }
>=20
> 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=E2=80=99ll poke aroun=
d. I=E2=80=99ll also try to figure out why I didn=E2=80=99t catch this in te=
sting.=