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[PATCH] fuzz: fix sparse memory access in the DMA callback


From: Alexander Bulekov
Subject: [PATCH] fuzz: fix sparse memory access in the DMA callback
Date: Tue, 6 Jul 2021 12:17:35 -0400

The code mistakenly relied on address_space_translate to store the
length remaining until the next memory-region. We care about this
because when there is RAM or sparse-memory neighboring on an MMIO
region, we should only write up to the border, to prevent inadvertently
invoking MMIO handlers within the DMA callback.

However address_space_translate_internal only stores the length until
the end of the MemoryRegion if memory_region_is_ram(mr). Otherwise
the *len is left unmodified. This caused some false-positive issues,
where the fuzzer found a way to perform a nested MMIO write through a
DMA callback on an [address, length] that started within sparse memory
and spanned some device MMIO regions.

To fix this, write to sparse memory in small chunks of
memory_access_size (similar to the underlying address_space_write code),
which will prevent accidentally hitting MMIO handlers through large
writes.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 tests/qtest/fuzz/generic_fuzz.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c
index 59be7219de..3e8ce29227 100644
--- a/tests/qtest/fuzz/generic_fuzz.c
+++ b/tests/qtest/fuzz/generic_fuzz.c
@@ -240,10 +240,17 @@ void fuzz_dma_read_cb(size_t addr, size_t len, 
MemoryRegion *mr)
                                       addr, &addr1, &l, true,
                                       MEMTXATTRS_UNSPECIFIED);
 
-        if (!(memory_region_is_ram(mr1) ||
-              memory_region_is_romd(mr1)) && mr1 != sparse_mem_mr) {
+        /*
+         *  If mr1 isn't RAM, address_space_translate doesn't update l. Use
+         *  memory_access_size to identify the number of bytes that it is safe
+         *  to write without accidentally writing to another MemoryRegion.
+         */
+        if (!memory_region_is_ram(mr1)) {
             l = memory_access_size(mr1, l, addr1);
-        } else {
+        }
+        if (memory_region_is_ram(mr1) ||
+            memory_region_is_romd(mr1) ||
+            mr1 == sparse_mem_mr) {
             /* ROM/RAM case */
             if (qtest_log_enabled) {
                 /*
-- 
2.28.0




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