[tip: x86/urgent] x86/entry: Remove UNTRAIN_RET from native_irq_return_ldt

[tip-bot2 for Alexandre Chartre]

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

Commit-ID:     d16e0b26672066035439b2f49887f6576c4a3689
Gitweb:        https://git.kernel.org/tip/d16e0b26672066035439b2f49887f6576c4a3689
Author:        Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
AuthorDate:    Wed, 13 Jul 2022 21:58:08 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Thu, 14 Jul 2022 09:45:12 +02:00

x86/entry: Remove UNTRAIN_RET from native_irq_return_ldt

UNTRAIN_RET is not needed in native_irq_return_ldt because RET
untraining has already been done at this point.

In addition, when the RETBleed mitigation is IBPB, UNTRAIN_RET clobbers
several registers (AX, CX, DX) so here it trashes user values which are
in these registers.

Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lore.kernel.org/r/35b0d50f-12d1-10c3-f5e8-d6c140486d4a@xxxxxxxxxx
---
 arch/x86/entry/entry_64.S | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 285e043..9953d96 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -727,7 +727,6 @@ native_irq_return_ldt:
 	pushq	%rdi				/* Stash user RDI */
 	swapgs					/* to kernel GS */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi	/* to kernel CR3 */
-	UNTRAIN_RET
 
 	movq	PER_CPU_VAR(espfix_waddr), %rdi
 	movq	%rax, (0*8)(%rdi)		/* user RAX */