[PATCH v4] riscv: entry: set a0 = -ENOSYS only when syscall != -1

[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 commit 9c2598d43510 ("riscv: entry:
Save a0 prior syscall_enter_from_user_mode()").

After analysing code, we think that regs->a0 = -ENOSYS should only be executed
when syscall != -1 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
executed. 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>
Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
---

v3 -> v4: use long instead of ulong to reduce type cast and avoid
          implementation-defined behavior, and make the judgment of syscall
          invalid more explicit
v2 -> v3: use if-statement instead of set default value,
          clarify the type of syscall
v1 -> v2: added explanation on why always got ENOSYS

 arch/riscv/kernel/traps.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..729f79c97e2bf 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -297,7 +297,7 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
-		ulong syscall = regs->a7;
+		long syscall = regs->a7;
 
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
@@ -306,9 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		syscall = syscall_enter_from_user_mode(regs, syscall);
 
-		if (syscall < NR_syscalls)
+		if (syscall >= 0 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
+		else if (syscall != -1)
 			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
-- 
2.41.0