[SRU][mantic:linux-gcp, lunar:linux-gcp][PATCH 1/2] x86/sev-es: Allow copy_from_kernel_nofault in earlier boot

[John Cabaj]

From: Adam Dunlap <acdunlap at google.com>

BugLink: https://bugs.launchpad.net/bugs/2037316

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 at linux.intel.com>
Signed-off-by: Adam Dunlap <acdunlap at google.com>
Link: https://lore.kernel.org/lkml/20230912002703.3924521-2-acdunlap@google.com/
Signed-off-by: John Cabaj <john.cabaj at canonical.com>
---
 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 5a53c2cc169c..6993f026adec 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)
-- 
2.34.1