Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for tdx_enc_status_changed()

Kirill A. Shutemov
2023-05-25T19:51:06Z

On Wed, May 24, 2023 at 02:28:51AM +0300, kirill.shutemov@linux.intel.com wrote:
> On Tue, May 23, 2023 at 03:43:15PM -0700, Dave Hansen wrote:
> > On 5/23/23 15:37, kirill.shutemov@linux.intel.com wrote:
> > >> How does this work with load_unaligned_zeropad()? Couldn't it be
> > >> running around poking at one of these vmalloc()'d pages via the direct
> > >> map during a shared->private conversion before the page has been accepted?
> > > Alias processing in __change_page_attr_set_clr() will change direct
> > > mapping if you call it on vmalloc()ed memory. I think we are safe wrt
> > > load_unaligned_zeropad() here.
> >
> > We're *eventually* OK:
> >
> > > /* Notify hypervisor that we are about to set/clr encryption attribute. */
> > > x86_platform.guest.enc_status_change_prepare(addr, numpages, enc);
> > >
> > > ret = __change_page_attr_set_clr(&cpa, 1);
> >
> > But what about in the middle between enc_status_change_prepare() and
> > __change_page_attr_set_clr()? Don't the direct map and the
> > shared/private status of the page diverge in there?
>
> Hmm. Maybe we would need to go through making the range in direct mapping
> non-present before notifying VMM about the change.
>
> I need to look at this again in the morning.

Okay, I've got around to it finally.

Private->Shared conversion is safe: we first set shared bit in the direct
mapping and all aliases and then call MapGPA enc_status_change_finish().
So we don't have privately mapped pages that we converted to shared with
MapGPA.

Shared->Private is not safe. As with Private->Shared, we adjust direct
mapping before notifying VMM and accepting the memory, so there's short
window when privately mapped memory that is neither mapped into SEPT nor
accepted. It is a problem as it can race with load_unaligned_zeropad().

Shared->Private conversion is rare. I only see one call total during the
boot in my setup. Worth fixing anyway.

The patch below fixes the issue by hooking up enc_status_change_prepare()
and doing conversion from Shared to Private there.

enc_status_change_finish() only covers Private to Shared conversion.

The patch is on top of unaccepted memory patchset. It is more convenient
base. I will rebase to Linus' tree if the approach looks sane to you.

Any comments?

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 32501277ef84..b73ec2449c64 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -713,16 +713,32 @@ static bool tdx_cache_flush_required(void)
return true;
}

+static bool tdx_enc_status_change_prepare(unsigned long vaddr, int numpages,
+ bool enc)
+{
+ phys_addr_t start = __pa(vaddr);
+ phys_addr_t end = __pa(vaddr + numpages * PAGE_SIZE);
+
+ if (!enc)
+ return true;
+
+ return tdx_enc_status_changed_phys(start, end, enc);
+}
+
/*
* Inform the VMM of the guest's intent for this physical page: shared with
* the VMM or private to the guest. The VMM is expected to change its mapping
* of the page in response.
*/
-static bool tdx_enc_status_changed(unsigned long vaddr, int numpages, bool enc)
+static bool tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
+ bool enc)
{
phys_addr_t start = __pa(vaddr);
phys_addr_t end = __pa(vaddr + numpages * PAGE_SIZE);

+ if (enc)
+ return true;
+
return tdx_enc_status_changed_phys(start, end, enc);
}

@@ -753,9 +769,10 @@ void __init tdx_early_init(void)
*/
physical_mask &= cc_mask - 1;

- x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
- x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
- x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
+ x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
+ x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
+ x86_platform.guest.enc_status_change_prepare = tdx_enc_status_change_prepare;
+ x86_platform.guest.enc_status_change_finish = tdx_enc_status_change_finish;

pr_info("Guest detected\n");
}
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 88085f369ff6..1ca9701917c5 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -150,7 +150,7 @@ struct x86_init_acpi {
* @enc_cache_flush_required Returns true if a cache flush is needed before changing page encryption status
*/
struct x86_guest {
- void (*enc_status_change_prepare)(unsigned long vaddr, int npages, bool enc);
+ bool (*enc_status_change_prepare)(unsigned long vaddr, int npages, bool enc);
bool (*enc_status_change_finish)(unsigned long vaddr, int npages, bool enc);
bool (*enc_tlb_flush_required)(bool enc);
bool (*enc_cache_flush_required)(void);
diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
index d82f4fa2f1bf..64664311ac2b 100644
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -130,8 +130,8 @@ struct x86_cpuinit_ops x86_cpuinit = {

static void default_nmi_init(void) { };

-static void enc_status_change_prepare_noop(unsigned long vaddr, int npages, bool enc) { }
-static bool enc_status_change_finish_noop(unsigned long vaddr, int npages, bool enc) { return false; }
+static bool enc_status_change_prepare_noop(unsigned long vaddr, int npages, bool enc) { return true; }
+static bool enc_status_change_finish_noop(unsigned long vaddr, int npages, bool enc) { return true; }
static bool enc_tlb_flush_required_noop(bool enc) { return false; }
static bool enc_cache_flush_required_noop(void) { return false; }
static bool is_private_mmio_noop(u64 addr) {return false; }
diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index e0b51c09109f..4f95c449a406 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -319,7 +319,7 @@ static void enc_dec_hypercall(unsigned long vaddr, int npages, bool enc)
#endif
}

-static void amd_enc_status_change_prepare(unsigned long vaddr, int npages, bool enc)
+static bool amd_enc_status_change_prepare(unsigned long vaddr, int npages, bool enc)
{
/*
* To maintain the security guarantees of SEV-SNP guests, make sure
@@ -327,6 +327,8 @@ static void amd_enc_status_change_prepare(unsigned long vaddr, int npages, bool
*/
if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP) && !enc)
snp_set_memory_shared(vaddr, npages);
+
+ return true;
}

/* Return true unconditionally: return value doesn't matter for the SEV side */
diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 7159cf787613..b8f48ebe753c 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -2151,7 +2151,8 @@ static int __set_memory_enc_pgtable(unsigned long addr, int numpages, bool enc)
cpa_flush(&cpa, x86_platform.guest.enc_cache_flush_required());

/* Notify hypervisor that we are about to set/clr encryption attribute. */
- x86_platform.guest.enc_status_change_prepare(addr, numpages, enc);
+ if (!x86_platform.guest.enc_status_change_prepare(addr, numpages, enc))
+ return -EIO;

ret = __change_page_attr_set_clr(&cpa, 1);

--
Kiryl Shutsemau / Kirill A. Shutemov