Commit d62e4f2b authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

x86/bug: Fix BUG_FORMAT vs KASLR 



Encoding a relative NULL pointer doesn't work for KASLR, when the
whole kernel image gets shifted, the __bug_table and the target string
get shifted by the same amount and the relative offset is preserved.

However when the target is an absolute 0 value and the __bug_table
gets moved about, the end result in a pointer equivalent to
kaslr_offset(), not NULL.

Notably, this will generate SHN_UNDEF relocations, and Ard would
really like to not have those at all.

Use the empty string to denote no-string.

Suggested-by: default avatarLinus Torvalds <torvalds@linuxfoundation.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
parent 860238af

arch/x86/include/asm/bug.h

+6 −2
Original line number Diff line number Diff line
@@ -86,7 +86,7 @@ extern void __WARN_trap(struct bug_entry *bug, ...); 
#ifdef CONFIG_DEBUG_BUGVERBOSE_DETAILED

#define WARN_CONDITION_STR(cond_str) cond_str

#else

#define WARN_CONDITION_STR(cond_str) NULL

#define WARN_CONDITION_STR(cond_str) ""

#endif



#define _BUG_FLAGS(cond_str, ins, flags, extra)				\
@@ -103,8 +103,12 @@ do { \ 
} while (0)



#define ARCH_WARN_ASM(file, line, flags, size)				\

	".pushsection .rodata.str1.1, \"aMS\", @progbits, 1\n"		\

	"99:\n"								\

	"\t.string \"\"\n"						\

	".popsection\n"							\

	"1:\t " ASM_UD2 "\n"						\

	_BUG_FLAGS_ASM("0", file, line, flags, size, "")

	_BUG_FLAGS_ASM("99b", file, line, flags, size, "")



#else

lib/bug.c

+11 −1
Original line number Diff line number Diff line
@@ -144,7 +144,17 @@ static const char *bug_get_format(struct bug_entry *bug) 
	const char *format = NULL;

#ifdef HAVE_ARCH_BUG_FORMAT

#ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS

	/*

	 * Allow an architecture to:

	 *  - relative encode NULL (difficult vs KASLR);

	 *  - use a literal 0 (there are no valid objects inside

	 *    the __bug_table itself to refer to after all);

	 *  - use an empty string.

	 */

	if (bug->format_disp)

		format = (const char *)&bug->format_disp + bug->format_disp;

	if (format && format[0] == '\0')

		format = NULL;

#else

	format = bug->format;

#endif