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 <[email protected]>
Co-developed-by: Ruizhe Pan <[email protected]>
Signed-off-by: Ruizhe Pan <[email protected]>
Co-developed-by: Shiqi Zhang <[email protected]>
Signed-off-by: Shiqi Zhang <[email protected]>
Signed-off-by: Celeste Liu <[email protected]>
Tested-by: Felix Yan <[email protected]>
Tested-by: Emil Renner Berthing <[email protected]>
Reviewed-by: Björn Töpel <[email protected]>
Reviewed-by: Guo Ren <[email protected]>
---
v4 -> v5: add Tested-by Emil Renner Berthing <[email protected]>
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
Hello:
This patch was applied to riscv/linux.git (fixes)
by Palmer Dabbelt <[email protected]>:
On Tue, 1 Aug 2023 22:15:16 +0800 you wrote:
> 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.
>
> [...]
Here is the summary with links:
- [v5] riscv: entry: set a0 = -ENOSYS only when syscall != -1
https://git.kernel.org/riscv/c/52449c17bdd1
You are awesome, thank you!
--
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html