On Wed, Feb 28, 2024 at 05:39:27PM -0800, Pawan Gupta wrote:
On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote:
On 27. 02. 24, 9:47, Nikolay Borisov wrote:
On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
Macro used for MDS mitigation executes VERW with relative addressing for
the operand. This is unnecessary and creates a problem for backports on
older kernels that don't support relocations in alternatives. Relocation
support was added by commit 270a69c4485d ("x86/alternative: Support
relocations in alternatives"). Also asm for fixed addressing is much
more cleaner than relative RIP addressing.
Simplify the asm by using fixed addressing for VERW operand.
Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
Reported-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@xxxxxxxx/
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/nospec-branch.h
b/arch/x86/include/asm/nospec-branch.h
index 2aa52cab1e46..ab19c7f1167b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,7 @@
* Note: Only the memory operand variant of VERW clears the CPU
buffers.
*/
.macro CLEAR_CPU_BUFFERS
- ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)),
X86_FEATURE_CLEAR_CPU_BUF
+ ALTERNATIVE "", __stringify(verw mds_verw_sel),
X86_FEATURE_CLEAR_CPU_BUF
Actually thinking about it more and discussing with Jiri (cc'ed), will
this work with KASLR enabled?
I might of course be wrong. We appear to rely on the asm+linker here.
You were right, with KASLR enabled, instructions with fixed addressing
in alternatives don't get relocated. I guess we will have to keep
rip-relative as is. Thanks for catching that.
Looks like this is not settled yet, it was naive of me to trust gdb on
/proc/kcore earlier with KASLR enabled.
With the below debug patch it appears the relocation with fixed
addresses is working as expected with KASLR enabled.