linux-kernel - Re: [PATCH] powerpc: Fix initrd corruption with relative jump labels

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Message-ID: <87pmwmsf9x.fsf@mpe.ellerman.id.au>
Date:   Wed, 16 Jun 2021 12:40:42 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Roman Bolshakov <r.bolshakov@...ro.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, dja@...ens.net,
        christophe.leroy@...roup.eu, a.kovaleva@...ro.com, groug@...d.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Fix initrd corruption with relative jump labels

Roman Bolshakov <r.bolshakov@...ro.com> writes:
> On Mon, Jun 14, 2021 at 11:14:40PM +1000, Michael Ellerman wrote:
>> Commit b0b3b2c78ec0 ("powerpc: Switch to relative jump labels") switched
>> us to using relative jump labels. That involves changing the code,
>> target and key members in struct jump_entry to be relative to the
>> address of the jump_entry, rather than absolute addresses.
>> 
>> We have two static inlines that create a struct jump_entry,
>> arch_static_branch() and arch_static_branch_jump(), as well as an asm
>> macro ARCH_STATIC_BRANCH, which is used by the pseries-only hypervisor
>> tracing code.
>> 
>> Unfortunately we missed updating the key to be a relative reference in
>> ARCH_STATIC_BRANCH.
>> 
>> That causes a pseries kernel to have a handful of jump_entry structs
>> with bad key values. Instead of being a relative reference they instead
>> hold the full address of the key.
>> 
>> However the code doesn't expect that, it still adds the key value to the
>> address of the jump_entry (see jump_entry_key()) expecting to get a
>> pointer to a key somewhere in kernel data.
>> 
>> The table of jump_entry structs sits in rodata, which comes after the
>> kernel text. In a typical build this will be somewhere around 15MB. The
>> address of the key will be somewhere in data, typically around 20MB.
>> Adding the two values together gets us a pointer somewhere around 45MB.
>> 
>> We then call static_key_set_entries() with that bad pointer and modify
>> some members of the struct static_key we think we are pointing at.
>> 
>> A pseries kernel is typically ~30MB in size, so writing to ~45MB won't
>> corrupt the kernel itself. However if we're booting with an initrd,
>> depending on the size and exact location of the initrd, we can corrupt
>> the initrd. Depending on how exactly we corrupt the initrd it can either
>> cause the system to not boot, or just corrupt one of the files in the
>> initrd.
>> 
>> The fix is simply to make the key value relative to the jump_entry
>> struct in the ARCH_STATIC_BRANCH macro.
>> 
>> Fixes: b0b3b2c78ec0 ("powerpc: Switch to relative jump labels")
>> Reported-by: Anastasia Kovaleva <a.kovaleva@...ro.com>
>> Reported-by: Roman Bolshakov <r.bolshakov@...ro.com>
>> Reported-by: Greg Kurz <groug@...d.org>
>> Reported-by: Daniel Axtens <dja@...ens.net>
>> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
>> ---
>>  arch/powerpc/include/asm/jump_label.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/powerpc/include/asm/jump_label.h b/arch/powerpc/include/asm/jump_label.h
>> index 2d5c6bec2b4f..93ce3ec25387 100644
>> --- a/arch/powerpc/include/asm/jump_label.h
>> +++ b/arch/powerpc/include/asm/jump_label.h
>> @@ -50,7 +50,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
>>  1098:	nop;					\
>>  	.pushsection __jump_table, "aw";	\
>>  	.long 1098b - ., LABEL - .;		\
>> -	FTR_ENTRY_LONG KEY;			\
>> +	FTR_ENTRY_LONG KEY - .;			\
>>  	.popsection
>>  #endif
>>  
>> -- 
>> 2.25.1
>> 
>
> Thanks for fixing the issue, Michael.
>
> Perhaps the fix should go to v5.13-rc7 if Linus plans to do it. It'd be
> good to avoid broken initrd on pseries guests in the release.

Yes it's in my fixes branch and I'll ask Linus to pull that before the
next rc.

cheers