From: Masahiro Yamada masahiroy@kernel.org
[ Upstream commit 8635e8df477bc77837886da206f4915576f88fec ]
This reverts commit cead61a6717a9873426b08d73a34a325e3546f5d.
It exported __stack_smash_handler and __guard, while they may not be defined by anyone.
The code *declares* __stack_smash_handler and __guard. It does not create weak symbols. If no external library is linked, they are left undefined, but yet exported.
If a loadable module tries to access non-existing symbols, bad things (a page fault, NULL pointer dereference, etc.) will happen. So, the current code is wrong and dangerous.
If the code were written as follows, it would *define* them as weak symbols so modules would be able to get access to them.
void (*__stack_smash_handler)(void *) __attribute__((weak)); EXPORT_SYMBOL(__stack_smash_handler);
long __guard __attribute__((weak)); EXPORT_SYMBOL(__guard);
In fact, modpost forbids exporting undefined symbols. It shows an error message if it detects such a mistake.
ERROR: modpost: "..." [...] was exported without definition
Unfortunately, it is checked only when the code is built as modular. The problem described above has been unnoticed for a long time because arch/um/os-Linux/user_syms.c is always built-in.
With a planned change in Kbuild, exporting undefined symbols will always result in a build error instead of a run-time error. It is a good thing, but we need to fix the breakage in advance.
One fix is to define weak symbols as shown above. An alternative is to export them conditionally as follows:
#ifdef CONFIG_STACKPROTECTOR extern void __stack_smash_handler(void *); EXPORT_SYMBOL(__stack_smash_handler);
external long __guard; EXPORT_SYMBOL(__guard); #endif
This is what other architectures do; EXPORT_SYMBOL(__stack_chk_guard) is guarded by #ifdef CONFIG_STACKPROTECTOR.
However, adding the #ifdef guard is not sensible because UML cannot enable the stack-protector in the first place! (Please note UML does not select HAVE_STACKPROTECTOR in Kconfig.)
So, the code is already broken (and unused) in multiple ways.
Just remove.
Signed-off-by: Masahiro Yamada masahiroy@kernel.org Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/um/os-Linux/user_syms.c | 7 ------- 1 file changed, 7 deletions(-)
diff --git a/arch/um/os-Linux/user_syms.c b/arch/um/os-Linux/user_syms.c index 9b62a9d352b3a..a310ae27b479a 100644 --- a/arch/um/os-Linux/user_syms.c +++ b/arch/um/os-Linux/user_syms.c @@ -37,13 +37,6 @@ EXPORT_SYMBOL(vsyscall_ehdr); EXPORT_SYMBOL(vsyscall_end); #endif
-/* Export symbols used by GCC for the stack protector. */ -extern void __stack_smash_handler(void *) __attribute__((weak)); -EXPORT_SYMBOL(__stack_smash_handler); - -extern long __guard __attribute__((weak)); -EXPORT_SYMBOL(__guard); - #ifdef _FORTIFY_SOURCE extern int __sprintf_chk(char *str, int flag, size_t len, const char *format); EXPORT_SYMBOL(__sprintf_chk);