linux-kernel - arch_set_user_pkey_access only works on the current task

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Message-ID: <b3762e3bc45d77869231271ffe0e259be118ad57.camel@pku.edu.cn>
Date:   Sat, 05 Jun 2021 21:10:28 +0800
From:   Jiashuo Liang <liangjs@....edu.cn>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org, Jiashuo Liang <liangjs@....edu.cn>
Subject: arch_set_user_pkey_access only works on the current task_struct

Hi,

I am learning the kernel implementation of the x86 PKU feature. I find the
arch_set_user_pkey_access function in arch/x86/kernel/fpu/xstate.c does not
use its first parameter. So it is perhaps a bug?

The arch_set_user_pkey_access function is supposed to set the PKRU register
for the task_struct specified by its first parameter tsk. But it is only
implemented for the current task_struct.

Fortunately, it has been called only with current task_struct in the kernel
code, so it appears to be okay. However, it can introduce bugs in the
future because people may expect it working on other task_struct.

This commit seems to be related: b79daf8589921 ("x86/mm/pkeys: Fix compact
mode by removing protection keys' XSAVE buffer manipulation").

Thank you!
liangjs