Re: [PATCH Part1 RFC v4 15/36] x86/mm: Add support to validate memory when changing C-bit

Borislav Petkov
2021-08-17T17:26:30Z

On Wed, Jul 07, 2021 at 01:14:45PM -0500, Brijesh Singh wrote:
> +struct __packed psc_hdr {
> + u16 cur_entry;
> + u16 end_entry;
> + u32 reserved;
> +};
> +
> +struct __packed psc_entry {
> + u64 cur_page : 12,
> + gfn : 40,
> + operation : 4,
> + pagesize : 1,
> + reserved : 7;
> +};
> +
> +struct __packed snp_psc_desc {
> + struct psc_hdr hdr;
> + struct psc_entry entries[VMGEXIT_PSC_MAX_ENTRY];
> +};

The majority of kernel code puts __packed after the struct definition,
let's put it there too pls, out of the way.

...

> +static int vmgexit_psc(struct snp_psc_desc *desc)
> +{
> + int cur_entry, end_entry, ret;
> + struct snp_psc_desc *data;
> + struct ghcb_state state;
> + struct ghcb *ghcb;
> + struct psc_hdr *hdr;
> + unsigned long flags;
> +
> + local_irq_save(flags);
> +
> + ghcb = __sev_get_ghcb(&state);
> + if (unlikely(!ghcb))
> + panic("SEV-SNP: Failed to get GHCB\n");
> +
> + /* Copy the input desc into GHCB shared buffer */
> + data = (struct snp_psc_desc *)ghcb->shared_buffer;
> + memcpy(ghcb->shared_buffer, desc, sizeof(*desc));
> +
> + hdr = &data->hdr;
> + cur_entry = hdr->cur_entry;
> + end_entry = hdr->end_entry;
> +
> + /*
> + * As per the GHCB specification, the hypervisor can resume the guest
> + * before processing all the entries. Checks whether all the entries
> + * are processed. If not, then keep retrying.
> + *
> + * The stragtegy here is to wait for the hypervisor to change the page
> + * state in the RMP table before guest access the memory pages. If the
> + * page state was not successful, then later memory access will result
> + * in the crash.
> + */
> + while (hdr->cur_entry <= hdr->end_entry) {
> + ghcb_set_sw_scratch(ghcb, (u64)__pa(data));
> +
> + ret = sev_es_ghcb_hv_call(ghcb, NULL, SVM_VMGEXIT_PSC, 0, 0);
> +
> + /*
> + * Page State Change VMGEXIT can pass error code through
> + * exit_info_2.
> + */
> + if (WARN(ret || ghcb->save.sw_exit_info_2,
> + "SEV-SNP: page state change failed ret=%d exit_info_2=%llx\n",
> + ret, ghcb->save.sw_exit_info_2))
> + return 1;

Yikes, you return here and below with interrupts disabled.

All your returns need to be "goto out;" instead where you do

out:
__sev_put_ghcb(&state);
local_irq_restore(flags);

Yap, you very likely need to put the GHCB too.

> + /*
> + * Lets do some sanity check that entry processing is not going
> + * backward. This will happen only if hypervisor is tricking us.
> + */
> + if (WARN((hdr->end_entry > end_entry) || (cur_entry > hdr->cur_entry),
> + "SEV-SNP: page state change processing going backward, end_entry "
> + "(expected %d got %d) cur_entry (expected %d got %d)\n",
> + end_entry, hdr->end_entry, cur_entry, hdr->cur_entry))
> + return 1;

WARNING: quoted string split across lines
#293: FILE: arch/x86/kernel/sev.c:750:
+ "SEV-SNP: page state change processing going backward, end_entry "
+ "(expected %d got %d) cur_entry (expected %d got %d)\n",

If you're wondering what to do, yes, you can really stretch that string
and shorten it too:

if (WARN((hdr->end_entry > end_entry) || (cur_entry > hdr->cur_entry),
"SEV-SNP: PSC processing going backwards, end_entry %d (got %d) cur_entry: %d (got %d)\n",
end_entry, hdr->end_entry, cur_entry, hdr->cur_entry))
return 1;

so that it fits on a single line and grepping can find it.

> + /* Lets verify that reserved bit is not set in the header*/
> + if (WARN(hdr->reserved, "Reserved bit is set in the PSC header\n"))

psc_entry has a ->reserved field too and since we're iterating over the
entries...

> + return 1;
> + }
> +
> + __sev_put_ghcb(&state);
> + local_irq_restore(flags);
> +
> + return 0;
> +}
> +
> +static void __set_page_state(struct snp_psc_desc *data, unsigned long vaddr,
> + unsigned long vaddr_end, int op)
> +{
> + struct psc_hdr *hdr;
> + struct psc_entry *e;
> + unsigned long pfn;
> + int i;
> +
> + hdr = &data->hdr;
> + e = data->entries;
> +
> + memset(data, 0, sizeof(*data));
> + i = 0;
> +
> + while (vaddr < vaddr_end) {
> + if (is_vmalloc_addr((void *)vaddr))
> + pfn = vmalloc_to_pfn((void *)vaddr);
> + else
> + pfn = __pa(vaddr) >> PAGE_SHIFT;
> +
> + e->gfn = pfn;
> + e->operation = op;
> + hdr->end_entry = i;
> +
> + /*
> + * The GHCB specification provides the flexibility to
> + * use either 4K or 2MB page size in the RMP table.
> + * The current SNP support does not keep track of the
> + * page size used in the RMP table. To avoid the
> + * overlap request, use the 4K page size in the RMP
> + * table.
> + */
> + e->pagesize = RMP_PG_SIZE_4K;
> +
> + vaddr = vaddr + PAGE_SIZE;
> + e++;
> + i++;
> + }
> +
> + /* Terminate the guest on page state change failure. */

That comment is kinda obvious :)

> + if (vmgexit_psc(data))
> + sev_es_terminate(1, GHCB_TERM_PSC);
> +}
> +
> +static void set_page_state(unsigned long vaddr, unsigned int npages, int op)
> +{
> + unsigned long vaddr_end, next_vaddr;
> + struct snp_psc_desc *desc;
> +
> + vaddr = vaddr & PAGE_MASK;
> + vaddr_end = vaddr + (npages << PAGE_SHIFT);
> +
> + desc = kmalloc(sizeof(*desc), GFP_KERNEL_ACCOUNT);

kzalloc() so that you don't have to memset() later in
__set_page_state().

> + if (!desc)
> + panic("failed to allocate memory");

Make that error message more distinctive so that *if* it happens, one
can pinpoint the place in the code where the panic comes from.

> + while (vaddr < vaddr_end) {
> + /*
> + * Calculate the last vaddr that can be fit in one
> + * struct snp_psc_desc.
> + */
> + next_vaddr = min_t(unsigned long, vaddr_end,
> + (VMGEXIT_PSC_MAX_ENTRY * PAGE_SIZE) + vaddr);
> +
> + __set_page_state(desc, vaddr, next_vaddr, op);
> +
> + vaddr = next_vaddr;
> + }
> +
> + kfree(desc);
> +}
> +

--
Regards/Gruss,
Boris.