Re: [PATCH] objtool,x86: Fix uaccess PUSHF/POPF validation

[Peter Zijlstra <peterz@infradead.org>]

Seems like I forgot LKML (again!)...

On Mon, Mar 08, 2021 at 03:46:04PM +0100, Peter Zijlstra wrote:
> 
> Commit ab234a260b1f ("x86/pv: Rework arch_local_irq_restore() to not
> use popf") replaced "push %reg; popf" with something like: "test
> $0x200, %reg; jz 1f; sti; 1:", which breaks the pushf/popf symmetry
> that commit ea24213d8088 ("objtool: Add UACCESS validation") relies
> on.
> 
> The result is:
> 
>   drivers/gpu/drm/amd/amdgpu/si.o: warning: objtool: si_common_hw_init()+0xf36: PUSHF stack exhausted
> 
> Meanwhile, commit c9c324dc22aa ("objtool: Support stack layout changes
> in alternatives") makes that we can actually use stack-ops in
> alternatives, which means we can revert 1ff865e343c2 ("x86,smap: Fix
> smap_{save,restore}() alternatives").
> 
> That in turn means we can limit the PUSHF/POPF handling of
> ea24213d8088 to those instructions that are in alternatives.
> 
> Fixes: ab234a260b1f ("x86/pv: Rework arch_local_irq_restore() to not use popf")
> Reported-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/include/asm/smap.h |   10 ++++------
>  tools/objtool/check.c       |    3 +++
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -58,9 +58,8 @@ static __always_inline unsigned long sma
>  	unsigned long flags;
>  
>  	asm volatile ("# smap_save\n\t"
> -		      ALTERNATIVE("jmp 1f", "", X86_FEATURE_SMAP)
> -		      "pushf; pop %0; " __ASM_CLAC "\n\t"
> -		      "1:"
> +		      ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC "\n\t",
> +				  X86_FEATURE_SMAP)
>  		      : "=rm" (flags) : : "memory", "cc");
>  
>  	return flags;
> @@ -69,9 +68,8 @@ static __always_inline unsigned long sma
>  static __always_inline void smap_restore(unsigned long flags)
>  {
>  	asm volatile ("# smap_restore\n\t"
> -		      ALTERNATIVE("jmp 1f", "", X86_FEATURE_SMAP)
> -		      "push %0; popf\n\t"
> -		      "1:"
> +		      ALTERNATIVE("", "push %0; popf\n\t",
> +				  X86_FEATURE_SMAP)
>  		      : : "g" (flags) : "memory", "cc");
>  }
>  
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2425,6 +2425,9 @@ static int handle_insn_ops(struct instru
>  		if (update_cfi_state(insn, next_insn, &state->cfi, op))
>  			return 1;
>  
> +		if (!insn->alt_group)
> +			continue;
> +
>  		if (op->dest.type == OP_DEST_PUSHF) {
>  			if (!state->uaccess_stack) {
>  				state->uaccess_stack = 1;