riscv: process: Fix kernel gp leakage

[PATCH] riscv: process: Fix kernel gp leakage

Posted by Stefan O'Rear 1 year, 10 months ago

childregs represents the registers which are active for the new thread
in user context. For a kernel thread, childregs->gp is never used since
the kernel gp is not touched by switch_to. For a user mode helper, the
gp value can be observed in user space after execve or possibly by other
means.

Fixes: 7db91e57a0ac ("RISC-V: Task implementation")
Signed-off-by: Stefan O'Rear <sorear@fastmail.com>
---
 arch/riscv/kernel/process.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 92922dbd5b5c..51042f48da17 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -27,8 +27,6 @@
 #include <asm/vector.h>
 #include <asm/cpufeature.h>
 
-register unsigned long gp_in_global __asm__("gp");
-
 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
 unsigned long __stack_chk_guard __read_mostly;
@@ -207,7 +205,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	if (unlikely(args->fn)) {
 		/* Kernel thread */
 		memset(childregs, 0, sizeof(struct pt_regs));
-		childregs->gp = gp_in_global;
 		/* Supervisor/Machine, irqs on: */
 		childregs->status = SR_PP | SR_PIE;
 
-- 
2.40.1

Re: [External] [PATCH] riscv: process: Fix kernel gp leakage

Posted by yunhui cui 1 year, 10 months ago

Hi Stefan,

On Wed, Mar 27, 2024 at 2:14 PM Stefan O'Rear <sorear@fastmail.com> wrote:
>
> childregs represents the registers which are active for the new thread
> in user context. For a kernel thread, childregs->gp is never used since
> the kernel gp is not touched by switch_to. For a user mode helper, the
> gp value can be observed in user space after execve or possibly by other
> means.
>
> Fixes: 7db91e57a0ac ("RISC-V: Task implementation")
> Signed-off-by: Stefan O'Rear <sorear@fastmail.com>
> ---
>  arch/riscv/kernel/process.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 92922dbd5b5c..51042f48da17 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -27,8 +27,6 @@
>  #include <asm/vector.h>
>  #include <asm/cpufeature.h>
>
> -register unsigned long gp_in_global __asm__("gp");
> -
>  #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
>  #include <linux/stackprotector.h>
>  unsigned long __stack_chk_guard __read_mostly;
> @@ -207,7 +205,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>         if (unlikely(args->fn)) {
>                 /* Kernel thread */
>                 memset(childregs, 0, sizeof(struct pt_regs));
> -               childregs->gp = gp_in_global;
>                 /* Supervisor/Machine, irqs on: */
>                 childregs->status = SR_PP | SR_PIE;
>
> --
> 2.40.1
>
>
Can you help express in more detail what the problem was before fixing it?

Thanks,
Yunhui