[PATCH v7 01/11] Revert "[PATCH] uml: export symbols added by GCC hardened"

Masahiro Yamada
2023-06-08T14:25:03Z

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. When the stack-protector is disabled, 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.

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>
---

Changes in v7:
- New patch

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 9b62a9d352b3..a310ae27b479 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);
--
2.39.2