[PATCH v2] fs: binfmt_elf_efpic: fix personality for ELF-FDPIC

[Greg Ungerer]

Hi Andrew,

On 18/9/23 05:26, Andrew Morton wrote:
> On Thu,  7 Sep 2023 11:18:08 +1000 Greg Ungerer <gerg at 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 at 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