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Date:   Mon, 18 Sep 2023 16:27:22 +1000
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Subject: Re: [PATCH v2] fs: binfmt_elf_efpic: fix personality for ELF-FDPIC
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To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-arm@lists.infradead.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        keescook@chromium.org, ebiederm@xmission.com, brauner@kernel.org,
        viro@zeniv.linux.org.uk
References: <20230907011808.2985083-1-gerg@kernel.org>
 <20230917122603.5466b1fc6d05ea9e9edee340@linux-foundation.org>
From:   Greg Ungerer <gerg@kernel.org>
In-Reply-To: <20230917122603.5466b1fc6d05ea9e9edee340@linux-foundation.org>
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Hi Andrew,

On 18/9/23 05:26, Andrew Morton wrote:
> On Thu,  7 Sep 2023 11:18:08 +1000 Greg Ungerer <gerg@kernel.org> wrote:
> 
>> The elf-fdpic loader hard sets the process personality to either
>> PER_LINUX_FDPIC for true elf-fdpic binaries or to PER_LINUX for
>> normal ELF binaries (in this case they would be constant displacement
>> compiled with -pie for example). The problem with that is that it
>> will lose any other bits that may be in the ELF header personality
>> (such as the "bug emulation" bits).
>>
>> On the ARM architecture the ADDR_LIMIT_32BIT flag is used to signify
>> a normal 32bit binary - as opposed to a legacy 26bit address binary.
>> This matters since start_thread() will set the ARM CPSR register as
>> required based on this flag. If the elf-fdpic loader loses this bit
>> the process will be mis-configured and crash out pretty quickly.
>>
>> Modify elf-fdpic loader personality setting so that it preserves the
>> upper three bytes by using the SET_PERSONALITY macro to set it. This
>> macro in the generic case sets PER_LINUX and preserves the upper bytes.
>> Architectures can override this for their specific use case, and ARM
>> does exactly this.
>>
>> The problem shows up quite easily running under qemu using the ARM
>> architecture, but not necessarily on all types of real ARM hardware.
>> If the underlying ARM processor does not support the legacy 26-bit
>> addressing mode then everything will work as expected.
> 
> I'm thinking
> 
> Fixes: 1bde925d23547 ("fs/binfmt_elf_fdpic.c: provide NOMMU loader for regular ELF binaries")
> Cc: <stable@vger.kernel.org>

Yes, that seems reasonable. It will apply easily, and legitimately fix this
specific issue going back to the original change.

Regards
Greg