[PATCH 0/4] kvm: sev: Fix issues reported by Sashiko

[PATCH 0/4] kvm: sev: Fix issues reported by Sashiko

[Jörg Rödel]

From: Joerg Roedel <joerg.roedel@amd.com>

Hi,

On the post of my direct-VMSA patch-set Sashiko reported a few real
pre-existing issues in the SEV-SNP launch_update code. This patch-set
addresses three of them:

	* Fix user-triggerable WARN_ON on LAUNCH_UPDATE path.

	* Check that CPUID pages are writable before writing error
	  information to it.

	* Fix kunmap_local() order.

Please review.

-Joerg

Joerg Roedel (4):
  kvm: sev: Fix user-space triggerable WARN_ON on snp_launch_update path
  kvm: sev: Unmap pages in correct order in sev_gmem_post_populate()
  KVM: guest_memfd: Add `write` parameter to kvm_gmem_populate()
  kvm: sev: Acquire a writeable page reference for CPUID pages

 arch/x86/kvm/svm/sev.c   | 15 +++++++++++++--
 arch/x86/kvm/vmx/tdx.c   |  2 +-
 include/linux/kvm_host.h |  4 +++-
 virt/kvm/guest_memfd.c   |  4 ++--
 4 files changed, 19 insertions(+), 6 deletions(-)

-- 
2.53.0

[PATCH 1/4] kvm: sev: Fix user-space triggerable WARN_ON on snp_launch_update path

[Jörg Rödel]

From: Joerg Roedel <joerg.roedel@amd.com>

Sashiko reported on an unrelated patch:

  [Severity: High]
  This is a pre-existing issue, but can a host userspace process trigger a
  kernel warning by passing a NULL user address (uaddr = 0) here?

  If params.uaddr is 0, src becomes NULL and passes the PAGE_ALIGNED(src)
  check. kvm_gmem_populate() skips fetching the user page and passes
  src_page = NULL to sev_gmem_post_populate().

  That function then unconditionally evaluates:

  WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO &&
               !src_page)

  Since the type isn't ZERO, won't this allow an unprivileged user to spam
  the kernel log?

The assessment is correct, so check for this condition earlier in the
snp_launch_update() path to avoid the WARN_ON_ONCE.

Fixes: dee5a47cc7a45 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command")
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
---
 arch/x86/kvm/svm/sev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 6c6a6d663e29..41dcba5180ca 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2438,6 +2438,13 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (!PAGE_ALIGNED(src))
 		return -EINVAL;
 
+	/*
+	 * Make sure user-mode did not pass NULL as src with
+	 * type != KVM_SEV_SNP_PAGE_TYPE_ZERO.
+	 */
+	if (src == NULL && params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO)
+		return -EINVAL;
+
 	npages = params.len / PAGE_SIZE;
 
 	/*
-- 
2.53.0

Re: [PATCH 1/4] kvm: sev: Fix user-space triggerable WARN_ON on snp_launch_update path

[Sean Christopherson]

Please capitalize the scope, i.e. "KVM: SEV:".

On Tue, Jun 23, 2026, Jörg Rödel wrote:
> From: Joerg Roedel <joerg.roedel@amd.com>
> 
> Sashiko reported on an unrelated patch:
> 
>   [Severity: High]
>   This is a pre-existing issue, but can a host userspace process trigger a
>   kernel warning by passing a NULL user address (uaddr = 0) here?
> 
>   If params.uaddr is 0, src becomes NULL and passes the PAGE_ALIGNED(src)
>   check. kvm_gmem_populate() skips fetching the user page and passes
>   src_page = NULL to sev_gmem_post_populate().
> 
>   That function then unconditionally evaluates:
> 
>   WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO &&
>                !src_page)
> 
>   Since the type isn't ZERO, won't this allow an unprivileged user to spam
>   the kernel log?

Use Reported-by: + Closes to capture Sashiko's effecitve bug report instead of
copy+pasting the finding.  There's no reason to treat Sashiko any differently
than any other bot.

> The assessment is correct, so check for this condition earlier in the
> snp_launch_update() path to avoid the WARN_ON_ONCE.
> 
> Fixes: dee5a47cc7a45 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command")
> Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
> ---
>  arch/x86/kvm/svm/sev.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 6c6a6d663e29..41dcba5180ca 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2438,6 +2438,13 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (!PAGE_ALIGNED(src))
>  		return -EINVAL;
>  
> +	/*
> +	 * Make sure user-mode did not pass NULL as src with
> +	 * type != KVM_SEV_SNP_PAGE_TYPE_ZERO.

Meh, that's pretty obvious from the code.

> +	 */
> +	if (src == NULL && params.type != KVM_SEV_SNP_PAGE_TYPE_ZERO)

I think I'd prefer this over checking for KVM_SEV_SNP_PAGE_TYPE_ZERO twice,
especially since the PAGE_ALIGNED() check for the NULL pointer case is rather
weird.

	if (params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO)
		src = NULL;
	else if (!params.uaddr || !PAGE_ALIGNED(params.uaddr))
		return -EINVAL;
	else
		src = u64_to_user_ptr(params.uaddr);


> +		return -EINVAL;

Gah, we created quite the mess for ourselves.  TDX returns -EOPNOTSUPP instead
of -EINVAL, I guess as a placeholder for in-place conversion?  I don't care which
error code is returned, but I do want KVM to be consistent.

We should also adjust TDX to pre-check the source address, because checking only
in the post-populate flow subtly relies on tdx_vcpu_init_mem_region() returning
immediately on error.  If that weren't the case (ignoring for the moment that
continuing on would be nonsensical), KVM would advace the address by PAGE_SIZE
and suddenly a NULL userspace address becomes non-NULL.

I also think it makes sense to drop the WARN in sev_gmem_post_populate(), it's
completely redundant once this bug is fixed.

Ugh, and both SNP and TDX fail to account for tags, and fail to check for
striding into kernel space.  Which I suppose is fine, since gup() handles those
correctly.  And I don't see a strong argument for disallowing tagged addresses,
because unlike the userspace address for memslots, KVM doesn't keep the address
around long-term.

So over two patches, the below?  I can send a v2, I've already got changelogs
written (I was fiddling around with extracting and reusing kvm_set_memory_region()'s
checks on the userspace address+size, but as above, convinced myself that KVM
should continue to allow tagged addresses for SNP and TDX).

---
 arch/x86/kvm/svm/sev.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 74fb15551e83..621a2eaa58f2 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2330,9 +2330,6 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	int level;
 	int ret;
 
-	if (WARN_ON_ONCE(sev_populate_args->type != KVM_SEV_SNP_PAGE_TYPE_ZERO && !src_page))
-		return -EINVAL;
-
 	ret = snp_lookup_rmpentry((u64)pfn, &assigned, &level);
 	if (ret || assigned) {
 		pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n",
@@ -2421,10 +2418,12 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	     params.type != KVM_SEV_SNP_PAGE_TYPE_CPUID))
 		return -EINVAL;
 
-	src = params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO ? NULL : u64_to_user_ptr(params.uaddr);
-
-	if (!PAGE_ALIGNED(src))
+	if (params.type == KVM_SEV_SNP_PAGE_TYPE_ZERO)
+		src = NULL;
+	else if (!params.uaddr || !PAGE_ALIGNED(params.uaddr))
 		return -EINVAL;
+	else
+		src = u64_to_user_ptr(params.uaddr);
 
 	npages = params.len / PAGE_SIZE;

---
 arch/x86/kvm/vmx/tdx.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index ffe9d0db58c5..b0ec054732b9 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3198,9 +3198,6 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	if (KVM_BUG_ON(kvm_tdx->page_add_src, kvm))
 		return -EIO;
 
-	if (!src_page)
-		return -EOPNOTSUPP;
-
 	kvm_tdx->page_add_src = src_page;
 	ret = kvm_tdp_mmu_map_private_pfn(arg->vcpu, gfn, pfn);
 	kvm_tdx->page_add_src = NULL;
@@ -3247,8 +3244,8 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 	if (copy_from_user(&region, u64_to_user_ptr(cmd->data), sizeof(region)))
 		return -EFAULT;
 
-	if (!PAGE_ALIGNED(region.source_addr) || !PAGE_ALIGNED(region.gpa) ||
-	    !region.nr_pages ||
+	if (!PAGE_ALIGNED(region.source_addr) || !region.source_addr ||
+	    !PAGE_ALIGNED(region.gpa) || !region.nr_pages ||
 	    region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa ||
 	    !vt_is_tdx_private_gpa(kvm, region.gpa) ||
 	    !vt_is_tdx_private_gpa(kvm, region.gpa + (region.nr_pages << PAGE_SHIFT) - 1))
-- 

[PATCH 2/4] kvm: sev: Unmap pages in correct order in sev_gmem_post_populate()

[Jörg Rödel]

From: Joerg Roedel <joerg.roedel@amd.com>

The kmap_local() interface requires unmapping of pages in reverse
order of mapping.

Fixes: 2a62345b3052 ("KVM: guest_memfd: GUP source pages prior to populating guest memory")
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
---
 arch/x86/kvm/svm/sev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 41dcba5180ca..f09d15f68964 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2360,8 +2360,8 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 
 		memcpy(dst_vaddr, src_vaddr, PAGE_SIZE);
 
-		kunmap_local(src_vaddr);
 		kunmap_local(dst_vaddr);
+		kunmap_local(src_vaddr);
 	}
 
 	ret = rmp_make_private(pfn, gfn << PAGE_SHIFT, PG_LEVEL_4K,
-- 
2.53.0

[PATCH 3/4] KVM: guest_memfd: Add `write` parameter to kvm_gmem_populate()

[Jörg Rödel]

From: Joerg Roedel <joerg.roedel@amd.com>

The call-path of kvm_gmem_populate() might subsequently write to the
page provided by user-space. This is used to provide detailed error
information in case the page population failed.

But since kvm_gmem_populate() only acquires a read-only reference to
the user-space page via get_user_pages_fast(), the error information
might be written to a read-only page later on.

Add a parameter to kvm_gmem_populate() to optionally acquire a
writeable reference to the source page to make sure page permissions
can be enforced.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
---
 arch/x86/kvm/svm/sev.c   | 2 +-
 arch/x86/kvm/vmx/tdx.c   | 2 +-
 include/linux/kvm_host.h | 4 +++-
 virt/kvm/guest_memfd.c   | 4 ++--
 4 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f09d15f68964..dab8109edf26 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2475,7 +2475,7 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	sev_populate_args.sev_fd = argp->sev_fd;
 	sev_populate_args.type = params.type;
 
-	count = kvm_gmem_populate(kvm, params.gfn_start, src, npages,
+	count = kvm_gmem_populate(kvm, params.gfn_start, src, npages, 0,
 				  sev_gmem_post_populate, &sev_populate_args);
 	if (count < 0) {
 		argp->error = sev_populate_args.fw_error;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 04ce321ebdf3..46b1d84fddf2 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3185,7 +3185,7 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 		};
 		gmem_ret = kvm_gmem_populate(kvm, gpa_to_gfn(region.gpa),
 					     u64_to_user_ptr(region.source_addr),
-					     1, tdx_gmem_post_populate, &arg);
+					     1, 0, tdx_gmem_post_populate, &arg);
 		if (gmem_ret < 0) {
 			ret = gmem_ret;
 			break;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4c14aee1fb06..622c0b04d8c3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2581,6 +2581,8 @@ int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_ord
  *       (passed to @post_populate, and incremented on each iteration
  *       if not NULL). Must be page-aligned.
  * @npages: number of pages to copy from userspace-buffer
+ * @write: user-space provided buffer must be writable. The function
+ *	 will acquire a writable reference when set to 1.
  * @post_populate: callback to issue for each gmem page that backs the GPA
  *                 range
  * @opaque: opaque data to pass to @post_populate callback
@@ -2597,7 +2599,7 @@ typedef int (*kvm_gmem_populate_cb)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 				    struct page *page, void *opaque);
 
 long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
-		       kvm_gmem_populate_cb post_populate, void *opaque);
+		       int write, kvm_gmem_populate_cb post_populate, void *opaque);
 #endif
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 69c9d6d546b2..7a245a402a1b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -859,7 +859,7 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
 }
 
 long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long npages,
-		       kvm_gmem_populate_cb post_populate, void *opaque)
+		       int write, kvm_gmem_populate_cb post_populate, void *opaque)
 {
 	struct kvm_memory_slot *slot;
 	int ret = 0;
@@ -893,7 +893,7 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 		if (src) {
 			unsigned long uaddr = (unsigned long)src + i * PAGE_SIZE;
 
-			ret = get_user_pages_fast(uaddr, 1, 0, &src_page);
+			ret = get_user_pages_fast(uaddr, 1, write, &src_page);
 			if (ret < 0)
 				break;
 			if (ret != 1) {
-- 
2.53.0

Re: [PATCH 3/4] KVM: guest_memfd: Add `write` parameter to kvm_gmem_populate()

[Sean Christopherson]

On Tue, Jun 23, 2026, Jörg Rödel wrote:
> From: Joerg Roedel <joerg.roedel@amd.com>
> 
> The call-path of kvm_gmem_populate() might subsequently write to the
> page provided by user-space. This is used to provide detailed error
> information in case the page population failed.
> 
> But since kvm_gmem_populate() only acquires a read-only reference to
> the user-space page via get_user_pages_fast(), the error information
> might be written to a read-only page later on.
> 
> Add a parameter to kvm_gmem_populate() to optionally acquire a
> writeable reference to the source page to make sure page permissions
> can be enforced.

Already fixed, commit f13e90059908 ("KVM: SEV: Pin source page for write when
adding CPUID data for SNP guest").

[PATCH 4/4] kvm: sev: Acquire a writeable page reference for CPUID pages

[Jörg Rödel]

From: Joerg Roedel <joerg.roedel@amd.com>

When the PSP checks on a user-provided CPUID page fail KVM will write
back the detailed error information to the user-provided buffer.

Make sure this buffer is actually writable to not write the errors to
a read-only page.

Fixes: 2a62345b3052 ("KVM: guest_memfd: GUP source pages prior to populating guest memory")
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
---
 arch/x86/kvm/svm/sev.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index dab8109edf26..5fd08d34be3f 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2415,6 +2415,7 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	struct kvm_memory_slot *memslot;
 	long npages, count;
 	void __user *src;
+	int write;
 
 	if (!sev_snp_guest(kvm) || !sev->snp_context)
 		return -EINVAL;
@@ -2475,7 +2476,10 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	sev_populate_args.sev_fd = argp->sev_fd;
 	sev_populate_args.type = params.type;
 
-	count = kvm_gmem_populate(kvm, params.gfn_start, src, npages, 0,
+	/* Acquire a write-reference for CPUID pages as kernel might write to it */
+	write = params.type == KVM_SEV_SNP_PAGE_TYPE_CPUID;
+
+	count = kvm_gmem_populate(kvm, params.gfn_start, src, npages, write,
 				  sev_gmem_post_populate, &sev_populate_args);
 	if (count < 0) {
 		argp->error = sev_populate_args.fw_error;
-- 
2.53.0