[tip: x86/mm] x86/sev-es: Allow copy_from_kernel_nofault() in earlier boot

[tip-bot2 for Adam Dunlap]

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

Commit-ID:     f79936545fb122856bd78b189d3c7ee59928c751
Gitweb:        https://git.kernel.org/tip/f79936545fb122856bd78b189d3c7ee59928c751
Author:        Adam Dunlap <acdunlap@xxxxxxxxxx>
AuthorDate:    Mon, 11 Sep 2023 17:27:02 -07:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 28 Sep 2023 22:49:35 +02:00

x86/sev-es: Allow copy_from_kernel_nofault() in earlier boot

Previously, if copy_from_kernel_nofault() was called before
boot_cpu_data.x86_virt_bits was set up, then it would trigger undefined
behavior due to a shift by 64.

This ended up causing boot failures in the latest version of ubuntu2204
in the gcp project when using SEV-SNP.

Specifically, this function is called during an early #VC handler which
is triggered by a CPUID to check if NX is implemented.

Fixes: 1aa9aa8ee517 ("x86/sev-es: Setup GHCB-based boot #VC handler")
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Adam Dunlap <acdunlap@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Tested-by: Jacob Xu <jacobhxu@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230912002703.3924521-2-acdunlap@xxxxxxxxxx
---
 arch/x86/mm/maccess.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
index 5a53c2c..6993f02 100644
--- a/arch/x86/mm/maccess.c
+++ b/arch/x86/mm/maccess.c
@@ -9,12 +9,21 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
 	unsigned long vaddr = (unsigned long)unsafe_src;
 
 	/*
-	 * Range covering the highest possible canonical userspace address
-	 * as well as non-canonical address range. For the canonical range
-	 * we also need to include the userspace guard page.
+	 * Do not allow userspace addresses.  This disallows
+	 * normal userspace and the userspace guard page:
 	 */
-	return vaddr >= TASK_SIZE_MAX + PAGE_SIZE &&
-	       __is_canonical_address(vaddr, boot_cpu_data.x86_virt_bits);
+	if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
+		return false;
+
+	/*
+	 * Allow everything during early boot before 'x86_virt_bits'
+	 * is initialized.  Needed for instruction decoding in early
+	 * exception handlers.
+	 */
+	if (!boot_cpu_data.x86_virt_bits)
+		return true;
+
+	return __is_canonical_address(vaddr, boot_cpu_data.x86_virt_bits);
 }
 #else
 bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)