x86/boot: Wrap literal addresses in absolute_pointer()

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/[email protected]/

Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

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! */

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));