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Re: [PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes table area |
Date: |
Tue, 2 Nov 2021 18:38:22 +0000 |
User-agent: |
Mutt/2.0.7 (2021-05-04) |
* Dov Murik (dovmurik@linux.ibm.com) wrote:
>
>
> On 02/11/2021 14:36, Dr. David Alan Gilbert wrote:
> > * Dov Murik (dovmurik@linux.ibm.com) wrote:
> >> Commit cff03145ed3c ("sev/i386: Introduce sev_add_kernel_loader_hashes
> >> for measured linux boot", 2021-09-30) introduced measured direct boot
> >> with -kernel, using an OVMF-designated hashes table which QEMU fills.
> >>
> >> However, no checks are performed on the validity of the hashes area
> >> designated by OVMF. Specifically, if OVMF publishes the
> >> SEV_HASH_TABLE_RV_GUID entry but it is filled with zeroes, this will
> >> cause QEMU to write the hashes entries over the first page of the
> >> guest's memory (GPA 0).
> >>
> >> Add validity checks to the published area. If the hashes table area's
> >> base address is zero, or its size is too small to fit the aligned hashes
> >> table, warn and skip the hashes entries addition. In such case, the
> >> following warning will be displayed:
> >>
> >> qemu-system-x86_64: warning: SEV: OVMF's hashes table area is invalid
> >> (base=0x0 size=0x0)
> >>
> >> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> >> Reported-by: Brijesh Singh <brijesh.singh@amd.com>
> >> ---
> >> target/i386/sev.c | 8 ++++++--
> >> 1 file changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/target/i386/sev.c b/target/i386/sev.c
> >> index 682b8ccf6c..a20ddb545e 100644
> >> --- a/target/i386/sev.c
> >> +++ b/target/i386/sev.c
> >> @@ -1201,13 +1201,18 @@ bool
> >> sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
> >> uint8_t kernel_hash[HASH_SIZE];
> >> uint8_t *hashp;
> >> size_t hash_len = HASH_SIZE;
> >> - int aligned_len;
> >> + int aligned_len = ROUND_UP(sizeof(SevHashTable), 16);
> >>
> >> if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) {
> >> warn_report("SEV: kernel specified but OVMF has no hash table
> >> guid");
> >> return false;
> >> }
> >> area = (SevHashTableDescriptor *)data;
> >> + if (!area->base || area->size < aligned_len) {
> >> + warn_report("SEV: OVMF's hashes table area is invalid (base=0x%x
> >> size=0x%x)",
> >> + area->base, area->size);
> >> + return false;
> >> + }
> >
> > That's probably a useful check, so
> >
> >
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >
>
> Thanks.
>
>
> > however, maybe it needs to be more thorough before using area->base to
> > qemu_map_ram_ptr? - I think it'll get unhappy if it's a bad address not
> > in a ram block. (Or check you're running over the end of a ramblock)
> >
>
> Does address_space_write perform these checks? Or maybe another API for
> accessing the guest's RAM?
I'm not sure; for example in address_space_map I don't see an check that
flatview_translate gives a valid mr.
Dave
> -Dov
>
> > Dave
> >
> >>
> >> /*
> >> * Calculate hash of kernel command-line with the terminating null
> >> byte. If
> >> @@ -1266,7 +1271,6 @@ bool
> >> sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
> >> memcpy(ht->kernel.hash, kernel_hash, sizeof(ht->kernel.hash));
> >>
> >> /* When calling sev_encrypt_flash, the length has to be 16 byte
> >> aligned */
> >> - aligned_len = ROUND_UP(ht->len, 16);
> >> if (aligned_len != ht->len) {
> >> /* zero the excess data so the measurement can be reliably
> >> calculated */
> >> memset(ht->padding, 0, aligned_len - ht->len);
> >> --
> >> 2.25.1
> >>
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
- [PATCH 1/3] sev/i386: Allow launching with -kernel if no OVMF hashes table found, (continued)
[PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes table area, Dov Murik, 2021/11/01
Re: [PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes table area, Daniel P . Berrangé, 2021/11/03
Re: [PATCH 0/3] SEV: fixes for -kernel launch with incompatible OVMF, Brijesh Singh, 2021/11/02
Re: [PATCH 0/3] SEV: fixes for -kernel launch with incompatible OVMF, Dov Murik, 2021/11/05