linux-kernel - [PATCH v8 4/5] x86/pkeys: Restore altstack access in sigreturn()

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Message-Id: <20240802061318.2140081-5-aruna.ramakrishna@oracle.com>
Date: Fri,  2 Aug 2024 06:13:17 +0000
From: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, dave.hansen@...ux.intel.com, tglx@...utronix.de,
        mingo@...nel.org, linux-mm@...ck.org, keith.lucas@...cle.com,
        jeffxu@...omium.org, rick.p.edgecombe@...el.com, jorgelo@...omium.org,
        keescook@...omium.org, sroettger@...gle.com, jannh@...gle.com,
        aruna.ramakrishna@...cle.com
Subject: [PATCH v8 4/5] x86/pkeys: Restore altstack access in sigreturn()

A process can disable access to the alternate signal stack by not
enabling the altstack's pkey in the PKRU register. Nevertheless, the
kernel updates the PKRU temporarily for signal handling. However, in
sigreturn(), restore_sigcontext() will restore the PKRU to the
user-defined PKRU value. This will cause restore_altstack() to fail with
a SIGSEGV as it needs read access to the altstack which is prohibited
by the user-defined PKRU value.

Fix this by restoring altstack before restoring PKRU.

Signed-off-by: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
---
 arch/x86/kernel/signal_64.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 8a94053c5444..ee9453891901 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -260,13 +260,13 @@ SYSCALL_DEFINE0(rt_sigreturn)
 
 	set_current_blocked(&set);
 
-	if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
+	if (restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	if (restore_signal_shadow_stack())
+	if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
 		goto badframe;
 
-	if (restore_altstack(&frame->uc.uc_stack))
+	if (restore_signal_shadow_stack())
 		goto badframe;
 
 	return regs->ax;
-- 
2.39.3