linux-kernel - Re: [PATCH] arm64: signal: nofpsimd: Do not allocate fp/simd context when not available

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Date:   Mon, 28 Feb 2022 08:56:49 +0100
From:   David Engraf <david.engraf@...go.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        mark.rutland@....com, elver@...gle.com, ebiederm@...ssion.com,
        seanjc@...gle.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: signal: nofpsimd: Do not allocate fp/simd context
 when not available

On 25.02.22 18:57, Mark Brown wrote:
> On Fri, Feb 25, 2022 at 11:40:08AM +0100, David Engraf wrote:
>> Commit 6d502b6ba1b2 ("arm64: signal: nofpsimd: Handle fp/simd context for
>> signal frames") introduced saving the fp/simd context for signal handling
>> only when support is available. But setup_sigframe_layout() always
>> reserves memory for fp/simd context. The additional memory is not touched
>> because preserve_fpsimd_context() is not called and thus the magic is
>> invalid.
>>
>> This may lead to an error when parse_user_sigframe() checks the fp/simd
>> area and does not find a valid magic number.
> 
> How did you spot this - do you have a system that can reproduce this?
> It'd be good to have coverage if there's testing but there's no easily
> obtainable userspace that I'm aware of.

I'm using a hypervisor which reports no fp/simd support. The user space 
was a busybox with init and shell. The shell gets a SIGSEGV because 
parse_user_sigframe() returns -EINVAL (bad magic).

user->sigframe in get_sigframe() uses the user stack pointer and the 
area was not zeroed. Thus the magic at fpsimd_offset is invalid.

Best regards
- David