Re: [PATCH v2] arm: Defer vetting of atags to setup.c

[Grant Likely]

On Wed, Jan 12, 2011 at 07:56:40PM +0000, Russell King - ARM Linux wrote:
> The patch looks good to me, except for one small concerning point, which
> I should've thought about earlier - sorry.
> 
> One of the things that __vet_atags does is to make sure that the data
> pointed to by 'r2' is actually an atag list.
> 
> Historically, we haven't required boot loaders to set r2 to anything -
> it used not to be defined to mean anything at all.  So with older boot
> loaders, this could be pointing at anything - it all depends what the
> boot loader code last did, and how the compiler optimized that code.
> 
> So, the __vet_atags asm tries dereferencing the values there after
> checking that it's word aligned.  We hope that we won't hit a read-
> sensitive device in doing so.  If we find a valid ATAG_CORE then we
> allow the pointer.  If we don't find those magic words, then we zero.
> 
> For boot loaders which always pass valid r2 values, moving this code
> after the MMU makes no difference.  For older boot loaders, it may be
> that __phys_to_virt(r2) ends up pointing at unmapped memory, which will
> cause a MMU abort - and as the vectors haven't been setup, that could
> hang the kernel.
> 
> So I think this has to stay in assembly code to make sure it works as
> required - to detect old boot loaders passing random values in r2 and
> allow the old method (atag address in machine_desc struct) to work.

Hmmm, I wondered about that.  I knew that code was to support older
firmware that doesn't pass proper atags, but from looking at the
explicit tests, it looked like the dereference would still be safe.

Oh well, it was worth a try.

g.

> 
> On Wed, Jan 12, 2011 at 11:03:07AM -0700, Grant Likely wrote:
> > Since the debug macros no longer depend on atag data, the vetting can
> > be deferred to setup_arch() in setup.c which simplifies the code
> > somewhat.
> > 
> > This patch removes __vet_atags() from head.S and moves it setup_arch().
> > I've tried to preserve the existing behaviour in this patch so the
> > extra atags vetting is only using when CONFIG_MMU is selected.
> > 
> > v2: Move removal of __machine_type_lookup to a separate patch.
> > 
> > Signed-off-by: Grant Likely <grant.likely@xxxxxxxxxxxx>
> > ---
> >  arch/arm/kernel/head-common.S |   51 +++++++----------------------------------
> >  arch/arm/kernel/head.S        |    1 -
> >  arch/arm/kernel/setup.c       |   16 +++++++++++++
> >  3 files changed, 25 insertions(+), 43 deletions(-)
> > 
> > diff --git a/arch/arm/kernel/head-common.S b/arch/arm/kernel/head-common.S
> > index c84b57d..8bb1829 100644
> > --- a/arch/arm/kernel/head-common.S
> > +++ b/arch/arm/kernel/head-common.S
> > @@ -11,50 +11,8 @@
> >   *
> >   */
> >  
> > -#define ATAG_CORE 0x54410001
> > -#define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)
> > -#define ATAG_CORE_SIZE_EMPTY ((2*4) >> 2)
> > -
> > -/*
> > - * Exception handling.  Something went wrong and we can't proceed.  We
> > - * ought to tell the user, but since we don't have any guarantee that
> > - * we're even running on the right architecture, we do virtually nothing.
> > - *
> > - * If CONFIG_DEBUG_LL is set we try to print out something about the error
> > - * and hope for the best (useful if bootloader fails to pass a proper
> > - * machine ID for example).
> > - */
> >  	__HEAD
> >  
> > -/* Determine validity of the r2 atags pointer.  The heuristic requires
> > - * that the pointer be aligned, in the first 16k of physical RAM and
> > - * that the ATAG_CORE marker is first and present.  Future revisions
> > - * of this function may be more lenient with the physical address and
> > - * may also be able to move the ATAGS block if necessary.
> > - *
> > - * Returns:
> > - *  r2 either valid atags pointer, or zero
> > - *  r5, r6 corrupted
> > - */
> > -__vet_atags:
> > -	tst	r2, #0x3			@ aligned?
> > -	bne	1f
> > -
> > -	ldr	r5, [r2, #0]			@ is first tag ATAG_CORE?
> > -	cmp	r5, #ATAG_CORE_SIZE
> > -	cmpne	r5, #ATAG_CORE_SIZE_EMPTY
> > -	bne	1f
> > -	ldr	r5, [r2, #4]
> > -	ldr	r6, =ATAG_CORE
> > -	cmp	r5, r6
> > -	bne	1f
> > -
> > -	mov	pc, lr				@ atag pointer is ok
> > -
> > -1:	mov	r2, #0
> > -	mov	pc, lr
> > -ENDPROC(__vet_atags)
> > -
> >  /*
> >   * The following fragment of code is executed with the MMU on in MMU mode,
> >   * and uses absolute addresses; this is not position independent.
> > @@ -158,6 +116,15 @@ __lookup_processor_type_data:
> >  	.long	__proc_info_end
> >  	.size	__lookup_processor_type_data, . - __lookup_processor_type_data
> >  
> > +/*
> > + * Exception handling.  Something went wrong and we can't proceed.  We
> > + * ought to tell the user, but since we don't have any guarantee that
> > + * we're even running on the right architecture, we do virtually nothing.
> > + *
> > + * If CONFIG_DEBUG_LL is set we try to print out something about the error
> > + * and hope for the best (useful if bootloader fails to pass a proper
> > + * machine ID for example).
> > + */
> >  __error_p:
> >  #ifdef CONFIG_DEBUG_LL
> >  	adr	r0, str_p1
> > diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> > index 19f3909..4b0cf4a 100644
> > --- a/arch/arm/kernel/head.S
> > +++ b/arch/arm/kernel/head.S
> > @@ -92,7 +92,6 @@ ENTRY(stext)
> >  	 * r1 = machine no, r2 = atags,
> >  	 * r9 = cpuid, r10 = procinfo
> >  	 */
> > -	bl	__vet_atags
> >  #ifdef CONFIG_SMP_ON_UP
> >  	bl	__fixup_smp
> >  #endif
> > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> > index 77266a8..47cf110 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -851,6 +851,22 @@ void __init setup_arch(char **cmdline_p)
> >  	if (mdesc->soft_reboot)
> >  		reboot_setup("s");
> >  
> > +#if defined(CONFIG_MMU)
> > +	/*
> > +	 * Determine validity of the atags pointer.  The heuristic requires
> > +	 * that the pointer be aligned, and that the ATAG_CORE marker is
> > +	 * first and present.
> > +	 */
> > +	if (__atags_pointer & 0x3)
> > +		__atags_pointer = 0;
> > +	if (__atags_pointer) {
> > +		struct tag *t = phys_to_virt(__atags_pointer);
> > +		if ((t->hdr.size != tag_size(tag_core)) &&
> > +		    (t->hdr.size != sizeof(struct tag_header)) &&
> > +		    (t->hdr.tag != ATAG_CORE))
> > +			__atags_pointer = 0;
> > +	}
> > +#endif
> >  	if (__atags_pointer)
> >  		tags = phys_to_virt(__atags_pointer);
> >  	else if (mdesc->boot_params)
> > 
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