linux-kernel - [PATCH v5 4/6] x86/tdx: Add a restriction on access to MMIO address

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Message-ID: <b9b120f5615776d3f557c96e244e57c97ac7d9d4.1724837158.git.legion@kernel.org>
Date: Wed, 28 Aug 2024 12:44:34 +0200
From: Alexey Gladkov <legion@...nel.org>
To: linux-kernel@...r.kernel.org,
	linux-coco@...ts.linux.dev
Cc: "Alexey Gladkov (Intel)" <legion@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yuan Yao <yuan.yao@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Yuntao Wang <ytcoode@...il.com>,
	Kai Huang <kai.huang@...el.com>,
	Baoquan He <bhe@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	cho@...rosoft.com,
	decui@...rosoft.com,
	John.Starks@...rosoft.com,
	stable@...r.kernel.org
Subject: [PATCH v5 4/6] x86/tdx: Add a restriction on access to MMIO address

From: "Alexey Gladkov (Intel)" <legion@...nel.org>

For security reasons, access from kernel space to MMIO addresses in
userspace should be restricted. All MMIO operations from kernel space
are considered trusted and are not validated.

For instance, if in response to a syscall, kernel does put_user() and
the target address is MMIO mapping in userspace, current #VE handler
threat this access as kernel MMIO which is wrong and have security
implications.

Cc: stable@...r.kernel.org
Signed-off-by: Alexey Gladkov (Intel) <legion@...nel.org>
---
 arch/x86/coco/tdx/tdx.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 5d2d07aa08ce..65f65015238a 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -411,6 +411,11 @@ static inline bool is_private_gpa(u64 gpa)
 	return gpa == cc_mkenc(gpa);
 }
 
+static inline bool is_kernel_addr(unsigned long addr)
+{
+	return (long)addr < 0;
+}
+
 static int get_phys_addr(unsigned long addr, phys_addr_t *phys_addr, bool *writable)
 {
 	unsigned int level;
@@ -606,6 +611,11 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	if (WARN_ON_ONCE(mmio == INSN_MMIO_DECODE_FAILED))
 		return -EINVAL;
 
+	if (!user_mode(regs) && !is_kernel_addr(ve->gla)) {
+		WARN_ONCE(1, "Access to userspace address is not supported");
+		return -EINVAL;
+	}
+
 	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
 
 	if (user_mode(regs)) {
-- 
2.46.0