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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: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
This commit is contained in:
Peter Zijlstra 2025-11-26 10:55:17 +01:00
parent 860238af7a
commit d62e4f2b95
2 changed files with 17 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
arch/x86/include/asm/bug.h
View File

@ -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

10
lib/bug.c
View File

@ -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