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

[Peter Zijlstra]

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@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  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;