Re: [PATCH] x86/static_call: Fix __static_call_fixup()

Peter Zijlstra
2023-08-15T23:16:12Z

On Wed, Aug 16, 2023 at 01:08:10AM +0200, Peter Zijlstra wrote:
>
> Christian reported spurious module crashes after some of Song's module

To clarify: module-load.

Obviously I shouldn't be writing Changelogs after 1am :-)

> memory layout patches.
>
> Turns out that if the very last instruction on the very last page of the
> module is a 'JMP __x86_return_thunk' then __static_call_fixup() will
> trip a fault and dies.
>
> And while the module rework made this slightly more likely to happen,
> it's always been possible.
>
> Fixes: ee88d363d156 ("x86,static_call: Use alternative RET encoding")
> Reported-by: Christian Bricart <christian@bricart.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> arch/x86/kernel/static_call.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> index b70670a98597..2e67512d7104 100644
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -186,6 +186,16 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
> */
> bool __static_call_fixup(void *tramp, u8 op, void *dest)
> {
> + /*
> + * Not all .return_sites are a static_call trampoline (most are not).
> + * Check if the next 3 bytes are still kernel text, if not, then this
> + * definitely is not a trampoline and we need not worry further.
> + *
> + * This avoids the memcmp() below tripping over pagefaults etc..
> + */
> + if (!kernel_text_address(tramp+7))
> + return false;
> +
> if (memcmp(tramp+5, tramp_ud, 3)) {
> /* Not a trampoline site, not our problem. */
> return false;