[PATCH v2] riscv: entry: set a0 prior to syscall_enter_from_user_mode

[Celeste Liu]

When we test seccomp with 6.4 kernel, we found errno has wrong value.
If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
get ENOSYS instead. We got same result with 9c2598d43510 ("riscv: entry: Save a0
prior syscall_enter_from_user_mode()").

After analysing code, we think that regs->a0 = -ENOSYS should be advanced before
syscall_enter_from_user_mode to fix this problem. In __seccomp_filter, when
seccomp rejected this syscall with specified errno, they will set a0 to return
number as syscall ABI, and then return -1. This return number is finally pass
as return number of syscall_enter_from_user_mode, and then is compared
with NR_syscalls after converted to ulong (so it will be ULONG_MAX).
The condition syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS
is always executable. It covered a0 set by seccomp, so we always get
ENOSYS when match seccomp RET_ERRNO rule.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reported-by: Felix Yan <felixonmars@xxxxxxxxxxxxx>
Co-developed-by: Ruizhe Pan <c141028@xxxxxxxxx>
Signed-off-by: Ruizhe Pan <c141028@xxxxxxxxx>
Co-developed-by: Shiqi Zhang <shiqi@xxxxxxxxxxxxxxxx>
Signed-off-by: Shiqi Zhang <shiqi@xxxxxxxxxxxxxxxx>
Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
Tested-by: Felix Yan <felixonmars@xxxxxxxxxxxxx>
---

v1 -> v2: added explanation on why always got ENOSYS

 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 f910dfccbf5d2..ccadb5ffd063c 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -301,6 +301,7 @@ 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);
 
@@ -308,8 +309,6 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
-			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
 	} else {
-- 
2.41.0