linux-kernel - Re: [PATCH bpf 3/3] selftest/bpf: Test the read of vsyscall page under x86-64

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Message-ID: <01ce6947-8b46-9e27-6ce2-ce5763e3c5a9@huaweicloud.com>
Date: Thu, 25 Jan 2024 15:54:21 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org, bpf@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
 linux-kernel@...r.kernel.org, xingwei lee <xrivendell7@...il.com>,
 Jann Horn <jannh@...gle.com>, houtao1@...wei.com
Subject: Re: [PATCH bpf 3/3] selftest/bpf: Test the read of vsyscall page
 under x86-64

On 1/23/2024 8:25 AM, Sohil Mehta wrote:
> On 1/18/2024 11:30 PM, Hou Tao wrote:
>
>> vsyscall page could be disabled by CONFIG_LEGACY_VSYSCALL_NONE or
>> vsyscall=none boot cmd-line, but it doesn't affect the reproduce of the
>> problem and the returned error codes.
>>
> With vsyscall=emulate a direct read of the vsyscall address from
> userspace is expected to go through. This is mode deprecated so maybe it
> wouldn't matter much. Without the fix in patch 2/3, do you see the same
> behavior with vsyscall=emulate set in the cmdline?

Er, I think it depends on whether or not SMAP [1] feature is available.
When SMAP feature is enabled, even the vsyscall page is populated,
reading the vsyscall page through bpf_read_kernel() will trigger a page
fault and then oops. But when there is not SMAP, bpf_read_kernel() will
succeed. So I think the test may need to be skipped if vsyscall_mode is
emulate.

[1]: https://en.wikipedia.org/wiki/Supervisor_Mode_Access_Prevention
>
> Sohil