83 lines
3.1 KiB
Diff
83 lines
3.1 KiB
Diff
From bd289293604d6f33e9fb89196f0b19117ce81f89 Mon Sep 17 00:00:00 2001
|
|
From: Paolo Bonzini <pbonzini@redhat.com>
|
|
Date: Wed, 20 Mar 2024 17:45:29 +0100
|
|
Subject: [PATCH 032/100] RAMBlock: make guest_memfd require uncoordinated
|
|
discard
|
|
|
|
RH-Author: Paolo Bonzini <pbonzini@redhat.com>
|
|
RH-MergeRequest: 245: SEV-SNP support
|
|
RH-Jira: RHEL-39544
|
|
RH-Acked-by: Thomas Huth <thuth@redhat.com>
|
|
RH-Acked-by: Bandan Das <bdas@redhat.com>
|
|
RH-Acked-by: Vitaly Kuznetsov <vkuznets@redhat.com>
|
|
RH-Commit: [32/91] 0c005849026c334737b88cbd20a0ac237dfca37e (bonzini/rhel-qemu-kvm)
|
|
|
|
Some subsystems like VFIO might disable ram block discard, but guest_memfd
|
|
uses discard operations to implement conversions between private and
|
|
shared memory. Because of this, sequences like the following can result
|
|
in stale IOMMU mappings:
|
|
|
|
1. allocate shared page
|
|
2. convert page shared->private
|
|
3. discard shared page
|
|
4. convert page private->shared
|
|
5. allocate shared page
|
|
6. issue DMA operations against that shared page
|
|
|
|
This is not a use-after-free, because after step 3 VFIO is still pinning
|
|
the page. However, DMA operations in step 6 will hit the old mapping
|
|
that was allocated in step 1.
|
|
|
|
Address this by taking ram_block_discard_is_enabled() into account when
|
|
deciding whether or not to discard pages.
|
|
|
|
Since kvm_convert_memory()/guest_memfd doesn't implement a
|
|
RamDiscardManager handler to convey and replay discard operations,
|
|
this is a case of uncoordinated discard, which is blocked/released
|
|
by ram_block_discard_require(). Interestingly, this function had
|
|
no use so far.
|
|
|
|
Alternative approaches would be to block discard of shared pages, but
|
|
this would cause guests to consume twice the memory if they use VFIO;
|
|
or to implement a RamDiscardManager and only block uncoordinated
|
|
discard, i.e. use ram_block_coordinated_discard_require().
|
|
|
|
[Commit message mostly by Michael Roth <michael.roth@amd.com>]
|
|
|
|
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
|
|
(cherry picked from commit 852f0048f3ea9f14de18eb279a99fccb6d250e8f)
|
|
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
|
|
---
|
|
system/physmem.c | 8 ++++++++
|
|
1 file changed, 8 insertions(+)
|
|
|
|
diff --git a/system/physmem.c b/system/physmem.c
|
|
index f5dfa20e57..5ebcf5be11 100644
|
|
--- a/system/physmem.c
|
|
+++ b/system/physmem.c
|
|
@@ -1846,6 +1846,13 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
|
|
assert(kvm_enabled());
|
|
assert(new_block->guest_memfd < 0);
|
|
|
|
+ if (ram_block_discard_require(true) < 0) {
|
|
+ error_setg_errno(errp, errno,
|
|
+ "cannot set up private guest memory: discard currently blocked");
|
|
+ error_append_hint(errp, "Are you using assigned devices?\n");
|
|
+ goto out_free;
|
|
+ }
|
|
+
|
|
new_block->guest_memfd = kvm_create_guest_memfd(new_block->max_length,
|
|
0, errp);
|
|
if (new_block->guest_memfd < 0) {
|
|
@@ -2109,6 +2116,7 @@ static void reclaim_ramblock(RAMBlock *block)
|
|
|
|
if (block->guest_memfd >= 0) {
|
|
close(block->guest_memfd);
|
|
+ ram_block_discard_require(false);
|
|
}
|
|
|
|
g_free(block);
|
|
--
|
|
2.39.3
|
|
|