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Date:   Tue, 14 Sep 2021 12:37:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v3 10/16] x86/extable: Mark handlers __cficanonical

On Tue, Sep 14, 2021 at 12:10:39PM -0700, Sami Tolvanen wrote:
> Exception tables are populated in assembly code, but the handlers are
> called in fixup_exception, which trips indirect call checking with
> CONFIG_CFI_CLANG. Mark the handlers __cficanonical to allow addresses
> taken in assembly to pass CFI checking.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/x86/mm/extable.c | 64 ++++++++++++++++++++++++-------------------
>  1 file changed, 36 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index e1664e9f969c..d16912dcbb4e 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -24,16 +24,18 @@ ex_fixup_handler(const struct exception_table_entry *x)
>  	return (ex_handler_t)((unsigned long)&x->handler + x->handler);
>  }
>  
> -__visible bool ex_handler_default(const struct exception_table_entry *fixup,
> -				  struct pt_regs *regs, int trapnr,
> -				  unsigned long error_code,
> -				  unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_default(const struct exception_table_entry *fixup,
> +			struct pt_regs *regs, int trapnr,
> +			unsigned long error_code,
> +			unsigned long fault_addr)
>  {
>  	regs->ip = ex_fixup_addr(fixup);
>  	return true;
>  }
>  EXPORT_SYMBOL(ex_handler_default);
>  
> +__visible __cficanonical
>  __visible bool ex_handler_fault(const struct exception_table_entry *fixup,

Double __visible here, but with that fixed:

Reviewed-by: Kees Cook <keescook@...omium.org>

I would note that given Linus's recent comments on attribute locations,
it does seem that __cficanonical is more a function behavior attribute
than a storage class... I'm not really sure:
https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com

-Kees

>  				struct pt_regs *regs, int trapnr,
>  				unsigned long error_code,
> @@ -55,10 +57,11 @@ EXPORT_SYMBOL_GPL(ex_handler_fault);
>   * of vulnerability by restoring from the initial state (essentially, zeroing
>   * out all the FPU registers) if we can't restore from the task's FPU state.
>   */
> -__visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
> -				    struct pt_regs *regs, int trapnr,
> -				    unsigned long error_code,
> -				    unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_fprestore(const struct exception_table_entry *fixup,
> +			  struct pt_regs *regs, int trapnr,
> +			  unsigned long error_code,
> +			  unsigned long fault_addr)
>  {
>  	regs->ip = ex_fixup_addr(fixup);
>  
> @@ -70,10 +73,11 @@ __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
>  }
>  EXPORT_SYMBOL_GPL(ex_handler_fprestore);
>  
> -__visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
> -				  struct pt_regs *regs, int trapnr,
> -				  unsigned long error_code,
> -				  unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_uaccess(const struct exception_table_entry *fixup,
> +			struct pt_regs *regs, int trapnr,
> +			unsigned long error_code,
> +			unsigned long fault_addr)
>  {
>  	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
>  	regs->ip = ex_fixup_addr(fixup);
> @@ -81,10 +85,11 @@ __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
>  }
>  EXPORT_SYMBOL(ex_handler_uaccess);
>  
> -__visible bool ex_handler_copy(const struct exception_table_entry *fixup,
> -			       struct pt_regs *regs, int trapnr,
> -			       unsigned long error_code,
> -			       unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_copy(const struct exception_table_entry *fixup,
> +		     struct pt_regs *regs, int trapnr,
> +		     unsigned long error_code,
> +		     unsigned long fault_addr)
>  {
>  	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
>  	regs->ip = ex_fixup_addr(fixup);
> @@ -93,10 +98,11 @@ __visible bool ex_handler_copy(const struct exception_table_entry *fixup,
>  }
>  EXPORT_SYMBOL(ex_handler_copy);
>  
> -__visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
> -				       struct pt_regs *regs, int trapnr,
> -				       unsigned long error_code,
> -				       unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
> +			     struct pt_regs *regs, int trapnr,
> +			     unsigned long error_code,
> +			     unsigned long fault_addr)
>  {
>  	if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
>  			 (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
> @@ -110,10 +116,11 @@ __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup
>  }
>  EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
>  
> -__visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
> -				       struct pt_regs *regs, int trapnr,
> -				       unsigned long error_code,
> -				       unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
> +			     struct pt_regs *regs, int trapnr,
> +			     unsigned long error_code,
> +			     unsigned long fault_addr)
>  {
>  	if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
>  			 (unsigned int)regs->cx, (unsigned int)regs->dx,
> @@ -126,10 +133,11 @@ __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup
>  }
>  EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
>  
> -__visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
> -				   struct pt_regs *regs, int trapnr,
> -				   unsigned long error_code,
> -				   unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
> +			 struct pt_regs *regs, int trapnr,
> +			 unsigned long error_code,
> +			 unsigned long fault_addr)
>  {
>  	if (static_cpu_has(X86_BUG_NULL_SEG))
>  		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
> -- 
> 2.33.0.309.g3052b89438-goog
> 

-- 
Kees Cook

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