linux-kernel - Re: [PATCH v2] riscv: entry: set a0 prior to syscall_enter_from_user

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Date:   Tue, 18 Jul 2023 21:35:37 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Celeste Liu <coelacanthushex@...il.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Björn Töpel 
        <bjorn@...osinc.com>, Conor Dooley <conor.dooley@...rochip.com>,
        linux-riscv@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org,
        Celeste Liu <CoelacanthusHex@...il.com>,
        Felix Yan <felixonmars@...hlinux.org>,
        Ruizhe Pan <c141028@...il.com>,
        Shiqi Zhang <shiqi@...c.iscas.ac.cn>
Subject: Re: [PATCH v2] riscv: entry: set a0 prior to
 syscall_enter_from_user_mode

Celeste Liu <coelacanthushex@...il.com> writes:

> 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.

Isn't something like...

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d..15a8b4898a6c 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;
@@ -306,7 +306,7 @@ 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 > -1 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else
 			regs->a0 = -ENOSYS;


...easier to read?


Björn