linux-kernel - Re: [PATCH v5 13/19] kasan: x86: Handle int3 for inline KASAN reports

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Message-ID: <epbqhjyfdt3daudp2wx54jsw6d7jf6ifbr3yknlfuqptz7b4uq@73n5k6b2jrrl>
Date: Mon, 8 Sep 2025 15:08:52 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Andrey Konovalov <andreyknvl@...il.com>
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Subject: Re: [PATCH v5 13/19] kasan: x86: Handle int3 for inline KASAN reports

On 2025-09-08 at 14:54:32 +0200, Maciej Wieczor-Retman wrote:
>On 2025-09-08 at 12:38:57 +0200, Maciej Wieczor-Retman wrote:
>>On 2025-09-06 at 19:19:01 +0200, Andrey Konovalov wrote:
>>>On Mon, Aug 25, 2025 at 10:30 PM Maciej Wieczor-Retman
>>>> diff --git a/arch/x86/mm/kasan_inline.c b/arch/x86/mm/kasan_inline.c
>>>> new file mode 100644
>>>> index 000000000000..9f85dfd1c38b
>>>> --- /dev/null
>>>> +++ b/arch/x86/mm/kasan_inline.c
>>>> @@ -0,0 +1,23 @@
>>>> +// SPDX-License-Identifier: GPL-2.0
>>>> +#include <linux/kasan.h>
>>>> +#include <linux/kdebug.h>
>>>> +
>>>> +bool kasan_inline_handler(struct pt_regs *regs)
>>>> +{
>>>> +       int metadata = regs->ax;
>>>> +       u64 addr = regs->di;
>>>> +       u64 pc = regs->ip;
>>>> +       bool recover = metadata & KASAN_RAX_RECOVER;
>>>> +       bool write = metadata & KASAN_RAX_WRITE;
>>>> +       size_t size = KASAN_RAX_SIZE(metadata);
>>>> +
>>>> +       if (user_mode(regs))
>>>> +               return false;
>>>> +
>>>> +       if (!kasan_report((void *)addr, size, write, pc))
>>>> +               return false;
>>>
>>>Hm, this part is different than on arm64: there, we don't check the
>>>return value.
>>>
>>>Do I understand correctly that the return value from this function
>>>controls whether we skip over the int3 instruction and continue the
>>>execution? If so, we should return the same value regardless of
>>>whether the report is suppressed or not. And then you should not need
>>>to explicitly check for KASAN_BIT_MULTI_SHOT in the latter patch.
>>
>>I recall there were some corner cases where this code path got called in outline
>>mode, didn't have a mismatch but still died due to the die() below. But I'll
>>recheck and either apply what you wrote above or get add a better explanation
>>to the patch message.
>
>Okay, so the int3_selftest_ip() is causing a problem in outline mode.
>
>I tried disabling kasan with kasan_disable_current() but thinking of it now it
>won't work because int3 handler will still be called and die() will happen.

Sorry, I meant to write that kasan_disable_current() works together with
if(!kasan_report()). Because without checking kasan_report()' return
value, if kasan is disabled through kasan_disable_current() it will have no
effect in both inline mode, and if int3 is called in outline mode - the
kasan_inline_handler will lead to die().

>
>What did you mean by "return the same value regardless of kasan_report()"? Then
>it will never reach the kasan_inline_recover() which I assume is needed for
>inline mode (once recover will work).
>
>-- 
>Kind regards
>Maciej Wieczór-Retman

-- 
Kind regards
Maciej Wieczór-Retman