[PATCH] x86/fpu: Fix AMD X86_BUG_FXSAVE_LEAK fixup

[PATCH] x86/fpu: Fix AMD X86_BUG_FXSAVE_LEAK fixup

[Uros Bizjak]

The assembly snippet in restore_fpregs_from_fpstate() that implements
X86_BUG_FXSAVE_LEAK fixup loads the value from a random variable,
preferably the one that is already in the L1 cache.

However, the access to fpinit_state via *fpstate pointer is not
implemented correctly. The "m" asm constraint requires dereferenced
pointer variable, otherwise the compiler just reloads the value
via temporary stack slot. The current asm code reflects this:

     mov    %rdi,(%rsp)
     ...
     fildl  (%rsp)

With dereferenced pointer variable, the code does what the
comment above the asm snippet says:

     fildl  (%rdi)

Also, remove the pointless %P operand modifier. The modifier is
ineffective on non-symbolic references - it was used to prevent
%rip-relative addresses in .altinstr sections, but FILDL in the
.text section can use %rip-relative addresses without problems.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 arch/x86/kernel/fpu/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 520deb411a70..1209c7aebb21 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -145,8 +145,8 @@ void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
 		asm volatile(
 			"fnclex\n\t"
 			"emms\n\t"
-			"fildl %P[addr]"	/* set F?P to defined value */
-			: : [addr] "m" (fpstate));
+			"fildl %[addr]"	/* set F?P to defined value */
+			: : [addr] "m" (*fpstate));
 	}
 
 	if (use_xsave()) {
-- 
2.44.0

[tip: x86/fpu] x86/fpu: Fix AMD X86_BUG_FXSAVE_LEAK fixup

[tip-bot2 for Uros Bizjak]

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

Commit-ID:     5d31174f3c8c465d9dbe88f6b9d1fe5716f44981
Gitweb:        https://git.kernel.org/tip/5d31174f3c8c465d9dbe88f6b9d1fe5716f44981
Author:        Uros Bizjak <ubizjak@gmail.com>
AuthorDate:    Fri, 15 Mar 2024 09:18:23 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 19 Mar 2024 14:02:29 +01:00

x86/fpu: Fix AMD X86_BUG_FXSAVE_LEAK fixup

The assembly snippet in restore_fpregs_from_fpstate() that implements
X86_BUG_FXSAVE_LEAK fixup loads the value from a random variable,
preferably the one that is already in the L1 cache.

However, the access to fpinit_state via *fpstate pointer is not
implemented correctly. The "m" asm constraint requires dereferenced
pointer variable, otherwise the compiler just reloads the value
via temporary stack slot. The current asm code reflects this:

     mov    %rdi,(%rsp)
     ...
     fildl  (%rsp)

With dereferenced pointer variable, the code does what the
comment above the asm snippet says:

     fildl  (%rdi)

Also, remove the pointless %P operand modifier. The modifier is
ineffective on non-symbolic references - it was used to prevent
%rip-relative addresses in .altinstr sections, but FILDL in the
.text section can use %rip-relative addresses without problems.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240315081849.5187-1-ubizjak@gmail.com
---
 arch/x86/kernel/fpu/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 520deb4..1209c7a 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -145,8 +145,8 @@ void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
 		asm volatile(
 			"fnclex\n\t"
 			"emms\n\t"
-			"fildl %P[addr]"	/* set F?P to defined value */
-			: : [addr] "m" (fpstate));
+			"fildl %[addr]"	/* set F?P to defined value */
+			: : [addr] "m" (*fpstate));
 	}
 
 	if (use_xsave()) {