On Feb 25, 2021, at 1:16 PM, Sean Christopherson [off-list ref] wrote:

On Wed, Feb 24, 2021, Nadav Amit wrote:

quoted

From: Nadav Amit <redacted>

Apparently, the assembly considers __ex_table as the location when the
pushsection directive was issued. Therefore when there is more than a
single entry in the vDSO exception table, the calculations of the base
and fixup are wrong.

Fix the calculations of the expected fault IP and new IP by adjusting
the base after each entry.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: x86@kernel.org
Signed-off-by: Nadav Amit <redacted>
---
arch/x86/entry/vdso/extable.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/vdso/extable.c b/arch/x86/entry/vdso/extable.c
index afcf5b65beef..c81e78636220 100644
--- a/arch/x86/entry/vdso/extable.c
+++ b/arch/x86/entry/vdso/extable.c

@@ -32,7 +32,7 @@ bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,

	nr_entries = image->extable_len / (sizeof(*extable));
	extable = image->extable;

-	for (i = 0; i < nr_entries; i++) {
+	for (i = 0; i < nr_entries; i++, base += sizeof(*extable)) {

It's been literally years since I wrote this code, but I distinctly remember the
addresses being relative to the base.  I also remember testing multiple entries,
but again, that was a long time ago.

Assuming things have changed, or I was flat out wrong, the comment above the
macro magic should also be updated.

/*
* Inject exception fixup for vDSO code.  Unlike normal exception fixup,
* vDSO uses a dedicated handler the addresses are relative to the overall
* exception table, not each individual entry.
*/

I will update the comment. I am not very familiar with pushsection stuff,
but the offsets were wrong.

Since you say you checked it, I wonder whether it can somehow be caused
by having exception table entries defined from multiple object files.

Anyhow, this change follows the kernel’s (not vDSO) exception table
scheme.