[PATCH 6.6 224/341] riscv: entry: always initialize regs->a0 to -ENOSYS

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6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Celeste Liu <coelacanthushex@xxxxxxxxx>

[ Upstream commit 61119394631f219e23ce98bcc3eb993a64a8ea64 ]

Otherwise when the tracer changes syscall number to -1, the kernel fails
to initialize a0 with -ENOSYS and subsequently fails to return the error
code of the failed syscall to userspace. For example, it will break
strace syscall tampering.

Fixes: 52449c17bdd1 ("riscv: entry: set a0 = -ENOSYS only when syscall != -1")
Reported-by: "Dmitry V. Levin" <ldv@xxxxxxxxx>
Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
Link: https://lore.kernel.org/r/20240627142338.5114-2-CoelacanthusHex@xxxxxxxxx
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/riscv/kernel/traps.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 67d0073fb624d..2158b7a65d74f 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -311,6 +311,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
+		regs->a0 = -ENOSYS;
 
 		riscv_v_vstate_discard(regs);
 
@@ -318,8 +319,6 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall >= 0 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else if (syscall != -1)
-			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
 	} else {
-- 
2.43.0







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