Re: [PATCH v2 4/6] arm64: probes: Add GCS support to bl/blr/ret

[Jeremy Linton <jeremy.linton@arm.com>]

Hi,

On 4/7/25 11:19 AM, Jeremy Linton wrote:
> The arm64 probe simulation doesn't currently have logic in place
> to deal with GCS and this results in core dumps if probes are inserted
> at control flow locations. Fix-up bl, blr and ret to manipulate the
> shadow stack as needed.
> 
> While we manipulate and validate the shadow stack correctly, the
> hardware provides additional security by only allowing GCS operations
> against pages which are marked to support GCS. For writing there is
> gcssttr() which enforces this, but there isn't an equivalent for
> reading. This means that uprobe users should be aware that probing on
> control flow instructions which require reading the shadow stack (ex:
> ret) offers lower security guarantees than what is achieved without
> the uprobe active.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
>   arch/arm64/kernel/probes/simulate-insn.c | 28 ++++++++++++++++++++----
>   1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c
> index 09a0b36122d0..1fc9bb69b1eb 100644
> --- a/arch/arm64/kernel/probes/simulate-insn.c
> +++ b/arch/arm64/kernel/probes/simulate-insn.c
> @@ -13,6 +13,7 @@
>   #include <asm/traps.h>
>   
>   #include "simulate-insn.h"
> +#include "asm/gcs.h"
>   
>   #define bbl_displacement(insn)		\
>   	sign_extend32(((insn) & 0x3ffffff) << 2, 27)
> @@ -49,6 +50,18 @@ static inline u32 get_w_reg(struct pt_regs *regs, int reg)
>   	return lower_32_bits(pt_regs_read_reg(regs, reg));
>   }
>   
> +static inline void update_lr(struct pt_regs *regs, long addr)
> +{
> +	int err = 0;
> +
> +	if (user_mode(regs) && task_gcs_el0_enabled(current)) {
> +		push_user_gcs(addr + 4,	 &err);
> +		if (err)
> +			force_sig(SIGSEGV);
> +	}
> +	procedure_link_pointer_set(regs, addr + 4);
> +}
> +
>   static bool __kprobes check_cbz(u32 opcode, struct pt_regs *regs)
>   {
>   	int xn = opcode & 0x1f;
> @@ -107,9 +120,8 @@ simulate_b_bl(u32 opcode, long addr, struct pt_regs *regs)
>   {
>   	int disp = bbl_displacement(opcode);
>   
> -	/* Link register is x30 */
>   	if (opcode & (1 << 31))
> -		set_x_reg(regs, 30, addr + 4);
> +		update_lr(regs, addr);
>   
>   	instruction_pointer_set(regs, addr + disp);
>   }
> @@ -133,17 +145,25 @@ simulate_br_blr(u32 opcode, long addr, struct pt_regs *regs)
>   	/* update pc first in case we're doing a "blr lr" */
>   	instruction_pointer_set(regs, get_x_reg(regs, xn));
>   
> -	/* Link register is x30 */
>   	if (((opcode >> 21) & 0x3) == 1)
> -		set_x_reg(regs, 30, addr + 4);
> +		update_lr(regs, addr);
>   }
>   
>   void __kprobes
>   simulate_ret(u32 opcode, long addr, struct pt_regs *regs)
>   {
> +	u64 ret_addr;
> +	int err = 0;
>   	int xn = (opcode >> 5) & 0x1f;
>   
>   	instruction_pointer_set(regs, get_x_reg(regs, xn));
> +
> +	if (user_mode(regs) && task_gcs_el0_enabled(current)) {
> +		ret_addr = pop_user_gcs(&err);
> +		if (err || ret_addr != procedure_link_pointer(regs))
> +			force_sig(SIGSEGV);
> +	}
> +
>   }

I was looking at this earlier this week and decided that this routine 
should have a return added following the force_sig(), and moving the 
instruction_pointer_set() below that. That syncs the behavior with what 
would happen without the uprobe active when ret_addr != LR. Then I 
dumped the 'xn' too.