Re: [PATCH v3 10/16] x86/extable: Mark handlers __cficanonical
From: Kees Cook
Date: Tue Sep 14 2021 - 15:37:45 EST
On Tue, Sep 14, 2021 at 12:10:39PM -0700, Sami Tolvanen wrote:
> Exception tables are populated in assembly code, but the handlers are
> called in fixup_exception, which trips indirect call checking with
> CONFIG_CFI_CLANG. Mark the handlers __cficanonical to allow addresses
> taken in assembly to pass CFI checking.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> ---
> arch/x86/mm/extable.c | 64 ++++++++++++++++++++++++-------------------
> 1 file changed, 36 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index e1664e9f969c..d16912dcbb4e 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -24,16 +24,18 @@ ex_fixup_handler(const struct exception_table_entry *x)
> return (ex_handler_t)((unsigned long)&x->handler + x->handler);
> }
>
> -__visible bool ex_handler_default(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_default(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> regs->ip = ex_fixup_addr(fixup);
> return true;
> }
> EXPORT_SYMBOL(ex_handler_default);
>
> +__visible __cficanonical
> __visible bool ex_handler_fault(const struct exception_table_entry *fixup,
Double __visible here, but with that fixed:
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
I would note that given Linus's recent comments on attribute locations,
it does seem that __cficanonical is more a function behavior attribute
than a storage class... I'm not really sure:
https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@xxxxxxxxxxxxxx
-Kees
> struct pt_regs *regs, int trapnr,
> unsigned long error_code,
> @@ -55,10 +57,11 @@ EXPORT_SYMBOL_GPL(ex_handler_fault);
> * of vulnerability by restoring from the initial state (essentially, zeroing
> * out all the FPU registers) if we can't restore from the task's FPU state.
> */
> -__visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_fprestore(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> regs->ip = ex_fixup_addr(fixup);
>
> @@ -70,10 +73,11 @@ __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
> }
> EXPORT_SYMBOL_GPL(ex_handler_fprestore);
>
> -__visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_uaccess(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
> regs->ip = ex_fixup_addr(fixup);
> @@ -81,10 +85,11 @@ __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
> }
> EXPORT_SYMBOL(ex_handler_uaccess);
>
> -__visible bool ex_handler_copy(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_copy(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
> regs->ip = ex_fixup_addr(fixup);
> @@ -93,10 +98,11 @@ __visible bool ex_handler_copy(const struct exception_table_entry *fixup,
> }
> EXPORT_SYMBOL(ex_handler_copy);
>
> -__visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
> (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
> @@ -110,10 +116,11 @@ __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup
> }
> EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
>
> -__visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
> (unsigned int)regs->cx, (unsigned int)regs->dx,
> @@ -126,10 +133,11 @@ __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup
> }
> EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
>
> -__visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
> - struct pt_regs *regs, int trapnr,
> - unsigned long error_code,
> - unsigned long fault_addr)
> +__visible __cficanonical
> +bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
> + struct pt_regs *regs, int trapnr,
> + unsigned long error_code,
> + unsigned long fault_addr)
> {
> if (static_cpu_has(X86_BUG_NULL_SEG))
> asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
> --
> 2.33.0.309.g3052b89438-goog
>
--
Kees Cook