[tip: x86/entry] x86/entry/64: Use TASK_SIZE_MAX for canonical RIP test

[tip-bot2 for Brian Gerst]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     58978b44df7276f7c75a2c6aad6c201421cd4daa
Gitweb:        https://git.kernel.org/tip/58978b44df7276f7c75a2c6aad6c201421cd4daa
Author:        Brian Gerst <brgerst@xxxxxxxxx>
AuthorDate:    Wed, 11 Oct 2023 18:43:50 -04:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 13 Oct 2023 13:05:28 +02:00

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@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
Link: https://lore.kernel.org/r/20231011224351.130935-3-brgerst@xxxxxxxxx
---
 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 9021465..4c7154d 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 the 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;
 
 	/*