linux-kernel - Re: [PATCH] x86/boot: Wrap literal addresses in absolute

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Message-ID: <202205182240.A8DDA7D4C@keescook>
Date:   Wed, 18 May 2022 22:41:42 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Guenter Roeck <linux@...ck-us.net>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/boot: Wrap literal addresses in absolute_pointer()

On Sun, Feb 27, 2022 at 11:59:18AM -0800, Kees Cook wrote:
> GCC 11 (incorrectly[1]) assumes that literal values cast to (void *)
> should be treated like a NULL pointer with an offset, and raises
> diagnostics when doing bounds checking under -Warray-bounds. GCC 12
> got "smarter" about finding these:
> 
> In function 'rdfs8',
>     inlined from 'vga_recalc_vertical' at /srv/code/arch/x86/boot/video-mode.c:124:29,
>     inlined from 'set_mode' at /srv/code/arch/x86/boot/video-mode.c:163:3:
> /srv/code/arch/x86/boot/boot.h:114:9: warning: array subscript 0 is outside array bounds of 'u8[0]' {aka 'unsigned char[]'} [-Warray-bounds]
>   114 |         asm volatile("movb %%fs:%1,%0" : "=q" (v) : "m" (*(u8 *)addr));
>       |         ^~~
> 
> This has been solved in other places[2] already by using the recently
> added absolute_pointer() macro. Do the same here.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
> [2] https://lore.kernel.org/all/20210912160149.2227137-1-linux@roeck-us.net/
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Guenter Roeck <linux@...ck-us.net>
> Cc: x86@...nel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Thread ping -- this is still needed for GCC 12. Can an x86 maintainer
either take this or Ack it and I'll carry it?

Thanks!

-Kees

> ---
>  arch/x86/boot/boot.h | 36 ++++++++++++++++++++++++------------
>  arch/x86/boot/main.c |  2 +-
>  2 files changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index 34c9dbb6a47d..686a9d75a0e4 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -110,66 +110,78 @@ typedef unsigned int addr_t;
>  
>  static inline u8 rdfs8(addr_t addr)
>  {
> +	u8 *ptr = (u8 *)absolute_pointer(addr);
>  	u8 v;
> -	asm volatile("movb %%fs:%1,%0" : "=q" (v) : "m" (*(u8 *)addr));
> +	asm volatile("movb %%fs:%1,%0" : "=q" (v) : "m" (*ptr));
>  	return v;
>  }
>  static inline u16 rdfs16(addr_t addr)
>  {
> +	u16 *ptr = (u16 *)absolute_pointer(addr);
>  	u16 v;
> -	asm volatile("movw %%fs:%1,%0" : "=r" (v) : "m" (*(u16 *)addr));
> +	asm volatile("movw %%fs:%1,%0" : "=r" (v) : "m" (*ptr));
>  	return v;
>  }
>  static inline u32 rdfs32(addr_t addr)
>  {
> +	u32 *ptr = (u32 *)absolute_pointer(addr);
>  	u32 v;
> -	asm volatile("movl %%fs:%1,%0" : "=r" (v) : "m" (*(u32 *)addr));
> +	asm volatile("movl %%fs:%1,%0" : "=r" (v) : "m" (*ptr));
>  	return v;
>  }
>  
>  static inline void wrfs8(u8 v, addr_t addr)
>  {
> -	asm volatile("movb %1,%%fs:%0" : "+m" (*(u8 *)addr) : "qi" (v));
> +	u8 *ptr = (u8 *)absolute_pointer(addr);
> +	asm volatile("movb %1,%%fs:%0" : "+m" (*ptr) : "qi" (v));
>  }
>  static inline void wrfs16(u16 v, addr_t addr)
>  {
> -	asm volatile("movw %1,%%fs:%0" : "+m" (*(u16 *)addr) : "ri" (v));
> +	u16 *ptr = (u16 *)absolute_pointer(addr);
> +	asm volatile("movw %1,%%fs:%0" : "+m" (*ptr) : "ri" (v));
>  }
>  static inline void wrfs32(u32 v, addr_t addr)
>  {
> -	asm volatile("movl %1,%%fs:%0" : "+m" (*(u32 *)addr) : "ri" (v));
> +	u32 *ptr = (u32 *)absolute_pointer(addr);
> +	asm volatile("movl %1,%%fs:%0" : "+m" (*ptr) : "ri" (v));
>  }
>  
>  static inline u8 rdgs8(addr_t addr)
>  {
> +	u8 *ptr = (u8 *)absolute_pointer(addr);
>  	u8 v;
> -	asm volatile("movb %%gs:%1,%0" : "=q" (v) : "m" (*(u8 *)addr));
> +	asm volatile("movb %%gs:%1,%0" : "=q" (v) : "m" (*ptr));
>  	return v;
>  }
>  static inline u16 rdgs16(addr_t addr)
>  {
> +	u16 *ptr = (u16 *)absolute_pointer(addr);
>  	u16 v;
> -	asm volatile("movw %%gs:%1,%0" : "=r" (v) : "m" (*(u16 *)addr));
> +	asm volatile("movw %%gs:%1,%0" : "=r" (v) : "m" (*ptr));
>  	return v;
>  }
>  static inline u32 rdgs32(addr_t addr)
>  {
> +	u32 *ptr = (u32 *)absolute_pointer(addr);
>  	u32 v;
> -	asm volatile("movl %%gs:%1,%0" : "=r" (v) : "m" (*(u32 *)addr));
> +	asm volatile("movl %%gs:%1,%0" : "=r" (v) : "m" (*ptr));
>  	return v;
>  }
>  
>  static inline void wrgs8(u8 v, addr_t addr)
>  {
> -	asm volatile("movb %1,%%gs:%0" : "+m" (*(u8 *)addr) : "qi" (v));
> +	u8 *ptr = (u8 *)absolute_pointer(addr);
> +	asm volatile("movb %1,%%gs:%0" : "+m" (*ptr) : "qi" (v));
>  }
>  static inline void wrgs16(u16 v, addr_t addr)
>  {
> -	asm volatile("movw %1,%%gs:%0" : "+m" (*(u16 *)addr) : "ri" (v));
> +	u16 *ptr = (u16 *)absolute_pointer(addr);
> +	asm volatile("movw %1,%%gs:%0" : "+m" (*ptr) : "ri" (v));
>  }
>  static inline void wrgs32(u32 v, addr_t addr)
>  {
> -	asm volatile("movl %1,%%gs:%0" : "+m" (*(u32 *)addr) : "ri" (v));
> +	u32 *ptr = (u32 *)absolute_pointer(addr);
> +	asm volatile("movl %1,%%gs:%0" : "+m" (*ptr) : "ri" (v));
>  }
>  
>  /* Note: these only return true/false, not a signed return value! */
> diff --git a/arch/x86/boot/main.c b/arch/x86/boot/main.c
> index e3add857c2c9..c421af5a3cdc 100644
> --- a/arch/x86/boot/main.c
> +++ b/arch/x86/boot/main.c
> @@ -33,7 +33,7 @@ static void copy_boot_params(void)
>  		u16 cl_offset;
>  	};
>  	const struct old_cmdline * const oldcmd =
> -		(const struct old_cmdline *)OLD_CL_ADDRESS;
> +		absolute_pointer(OLD_CL_ADDRESS);
>  
>  	BUILD_BUG_ON(sizeof(boot_params) != 4096);
>  	memcpy(&boot_params.hdr, &hdr, sizeof(hdr));
> -- 
> 2.32.0
> 

-- 
Kees Cook