diff mbox series

[2/6] x86/sev-es: Forward page-faults which happen during emulation

Message ID 20210512075445.18935-3-joro@8bytes.org
State Superseded
Headers show
Series [1/6] x86/sev-es: Don't return NULL from sev_es_get_ghcb() | expand

Commit Message

Joerg Roedel May 12, 2021, 7:54 a.m. UTC 
From: Joerg Roedel <jroedel@suse.de>

When emulating guest instructions for MMIO or IOIO accesses the #VC
handler might get a page-fault and will not be able to complete. Forward
the page-fault in this case to the correct handler instead of killing
the machine.

Fixes: 0786138c78e7 ("x86/sev-es: Add a Runtime #VC Exception Handler")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 4 ++++
 1 file changed, 4 insertions(+)

Comments

Joerg Roedel May 19, 2021, 1:16 p.m. UTC | #1 
Hi Sean,

On Wed, May 12, 2021 at 05:31:03PM +0000, Sean Christopherson wrote:
> This got me looking at the flows that "inject" #PF, and I'm pretty sure there

> are bugs in __vc_decode_user_insn() + insn_get_effective_ip().

> 

> Problem #1: __vc_decode_user_insn() assumes a #PF if insn_fetch_from_user_inatomic()

> fails, but the majority of failure cases in insn_get_seg_base() are #GPs, not #PF.

> 

> 	res = insn_fetch_from_user_inatomic(ctxt->regs, buffer);

> 	if (!res) {

> 		ctxt->fi.vector     = X86_TRAP_PF;

> 		ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;

> 		ctxt->fi.cr2        = ctxt->regs->ip;

> 		return ES_EXCEPTION;

> 	}

> 

> Problem #2: Using '0' as an error code means a legitimate effective IP of '0'

> will be misinterpreted as a failure.  Practically speaking, I highly doubt anyone

> will ever actually run code at address 0, but it's technically possible.  The

> most robust approach would be to pass a pointer to @ip and return an actual error

> code.  Using a non-canonical magic value might also work, but that could run afoul

> of future shenanigans like LAM.

> 

> 	ip = insn_get_effective_ip(regs);

> 	if (!ip)

> 		return 0;


Your observations are all correct. I put some changes onto this
patch-set to fix these problems.

Regards,

	Joerg
diff mbox series

Patch

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c49270c7669e..6530a844eb61 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1265,6 +1265,10 @@  static __always_inline void vc_forward_exception(struct es_em_ctxt *ctxt)
 	case X86_TRAP_UD:
 		exc_invalid_op(ctxt->regs);
 		break;
+	case X86_TRAP_PF:
+		write_cr2(ctxt->fi.cr2);
+		exc_page_fault(ctxt->regs, error_code);
+		break;
 	case X86_TRAP_AC:
 		exc_alignment_check(ctxt->regs, error_code);
 		break;