linux-kernel - [PATCH v3 2/3] x86/entry/64: Use TASK_SIZE

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Message-ID: <20231011224351.130935-3-brgerst@gmail.com>
Date:   Wed, 11 Oct 2023 18:43:50 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>
Subject: [PATCH v3 2/3] x86/entry/64: Use TASK_SIZE_MAX for canonical RIP test

Using shifts to determine if an address is canonical is difficult for
the compiler to optimize when the virtual address width is variable
(LA57 feature) without using inline assembly.  Instead, compare RIP
against TASK_SIZE_MAX.  The only user executable address outside of that
range is the deprecated vsyscall page, which can fall back to using IRET.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/entry/common.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 207149a0a9b3..e3d6f255379f 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -110,10 +110,10 @@ __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
 	 * in kernel space.  This essentially lets the user take over
 	 * the kernel, since userspace controls RSP.
 	 *
-	 * Change top bits to match most significant bit (47th or 56th bit
-	 * depending on paging mode) in the address.
+	 * TASK_SIZE_MAX covers all user-accessible addresses other than
+	 * the deprecated vsyscall page.
 	 */
-	if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHIFT + 1)))
+	if (unlikely(regs->ip >= TASK_SIZE_MAX))
 		return false;
 
 	/*
-- 
2.41.0