[tip: x86/shstk] x86/shstk: Check that SSP is aligned on sigreturn

[tip-bot2 for Rick Edgecombe]

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

Commit-ID:     580c301b703f720a18bf0a03cedf57f961d80eea
Gitweb:        https://git.kernel.org/tip/580c301b703f720a18bf0a03cedf57f961d80eea
Author:        Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
AuthorDate:    Mon, 12 Jun 2023 17:10:58 -07:00
Committer:     Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
CommitterDate: Tue, 11 Jul 2023 14:12:50 -07:00

x86/shstk: Check that SSP is aligned on sigreturn

The shadow stack signal frame is read by the kernel on sigreturn. It
relies on shadow stack memory protections to prevent forgeries of this
signal frame (which included the pre-signal SSP). It also relies on the
shadow stack signal frame to have bit 63 set. Since this bit would not be
set via typical shadow stack operations, so the kernel can assume it was a
value it placed there.

However, in order to support 32 bit shadow stack, the INCSSPD instruction
can increment the shadow stack by 4 bytes. In this case SSP might be
pointing to a region spanning two 8 byte shadow stack frames. It could
confuse the checks described above.

Since the kernel only supports shadow stack in 64 bit, just check that
the SSP is 8 byte aligned in the sigreturn path.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230613001108.3040476-33-rick.p.edgecombe%40intel.com
---
 arch/x86/kernel/shstk.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index f02e8ea..a8705f7 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -252,6 +252,9 @@ static int shstk_pop_sigframe(unsigned long *ssp)
 	unsigned long token_addr;
 	int err;
 
+	if (!IS_ALIGNED(*ssp, 8))
+		return -EINVAL;
+
 	err = get_shstk_data(&token_addr, (unsigned long __user *)*ssp);
 	if (unlikely(err))
 		return err;