[PATCH 2/5] x86/sev-es: Check if regs->sp is trusted before adjusting #VC IST stack

From: Joerg Roedel <joro@8bytes.org>
To: x86@kernel.org
Cc: Joerg Roedel <joro@8bytes.org>, Joerg Roedel <jroedel@suse.de>,
	Andy Lutomirski <luto@kernel.org>,
	stable@vger.kernel.org, hpa@zytor.com,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: [PATCH 2/5] x86/sev-es: Check if regs->sp is trusted before adjusting #VC IST stack
Date: Wed,  3 Mar 2021 15:17:13 +0100	[thread overview]
Message-ID: <20210303141716.29223-3-joro@8bytes.org> (raw)
In-Reply-To: <20210303141716.29223-1-joro@8bytes.org>

From: Joerg Roedel <jroedel@suse.de>

The code in the NMI handler to adjust the #VC handler IST stack is
needed in case an NMI hits when the #VC handler is still using its IST
stack.
But the check for this condition also needs to look if the regs->sp
value is trusted, meaning it was not set by user-space. Extend the
check to not use regs->sp when the NMI interrupted user-space code or
the SYSCALL gap.

Reported-by: Andy Lutomirski <luto@kernel.org>
Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
Cc: stable@vger.kernel.org # 5.10+
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev-es.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 1e78f4bd7bf2..28b0144daddd 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -121,8 +121,18 @@ static void __init setup_vc_stacks(int cpu)
 	cea_set_pte((void *)vaddr, pa, PAGE_KERNEL);
 }
 
-static __always_inline bool on_vc_stack(unsigned long sp)
+static __always_inline bool on_vc_stack(struct pt_regs *regs)
 {
+	unsigned long sp = regs->sp;
+
+	/* User-mode RSP is not trusted */
+	if (user_mode(regs))
+		return false;
+
+	/* SYSCALL gap still has user-mode RSP */
+	if (ip_within_syscall_gap(regs))
+		return false;
+
 	return ((sp >= __this_cpu_ist_bottom_va(VC)) && (sp < __this_cpu_ist_top_va(VC)));
 }
 
@@ -144,7 +154,7 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
 	old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
 
 	/* Make room on the IST stack */
-	if (on_vc_stack(regs->sp))
+	if (on_vc_stack(regs))
 		new_ist = ALIGN_DOWN(regs->sp, 8) - sizeof(old_ist);
 	else
 		new_ist = old_ist - sizeof(old_ist);
-- 
2.30.1

