For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com.

***

Subject: [PATCH] tracing/dma: Cap dma_map_sg tracepoint arrays to prevent buffer overflow
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

The dma_map_sg tracepoint can trigger a perf buffer overflow when
tracing large scatter-gather lists. With devices like virtio-gpu
creating large DRM buffers, nents can exceed 1000 entries, resulting
in:

  phys_addrs: 1000 * 8 bytes = 8,000 bytes
  dma_addrs:  1000 * 8 bytes = 8,000 bytes
  lengths:    1000 * 4 bytes = 4,000 bytes
  Total: ~20,000 bytes

This exceeds PERF_MAX_TRACE_SIZE (8192 bytes), causing:

  WARNING: CPU: 0 PID: 5497 at kernel/trace/trace_event_perf.c:405
  perf buffer not large enough, wanted 24620, have 8192

Cap all three dynamic arrays at 128 entries. This limits the total
event size to approximately 2,760 bytes, safely under the 8KB limit
while still providing sufficient debugging information for typical
cases.

When entries are truncated, users can see the actual counts in the
trace output to know the full extent of the operation.

Reported-by: syzbot+28cea38c382fd15e751a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=28cea38c382fd15e751a
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
---
 include/trace/events/dma.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
index b3fef140ae15..03def2ee47d8 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h
@@ -275,6 +275,8 @@ TRACE_EVENT(dma_free_sgt,
 				sizeof(u64), sizeof(u64)))
 );
 
+#define DMA_TRACE_MAX_ENTRIES 128
+
 TRACE_EVENT(dma_map_sg,
 	TP_PROTO(struct device *dev, struct scatterlist *sgl, int nents,
 		 int ents, enum dma_data_direction dir, unsigned long attrs),
@@ -282,9 +284,9 @@ TRACE_EVENT(dma_map_sg,
 
 	TP_STRUCT__entry(
 		__string(device, dev_name(dev))
-		__dynamic_array(u64, phys_addrs, nents)
-		__dynamic_array(u64, dma_addrs, ents)
-		__dynamic_array(unsigned int, lengths, ents)
+		__dynamic_array(u64, phys_addrs, min_t(int, nents, DMA_TRACE_MAX_ENTRIES))
+		__dynamic_array(u64, dma_addrs, min_t(int, ents, DMA_TRACE_MAX_ENTRIES))
+		__dynamic_array(unsigned int, lengths, min_t(int, ents, DMA_TRACE_MAX_ENTRIES))
 		__field(enum dma_data_direction, dir)
 		__field(unsigned long, attrs)
 	),
@@ -292,11 +294,13 @@ TRACE_EVENT(dma_map_sg,
 	TP_fast_assign(
 		struct scatterlist *sg;
 		int i;
+		int traced_nents = min_t(int, nents, DMA_TRACE_MAX_ENTRIES);
+		int traced_ents = min_t(int, ents, DMA_TRACE_MAX_ENTRIES);
 
 		__assign_str(device);
-		for_each_sg(sgl, sg, nents, i)
+		for_each_sg(sgl, sg, traced_nents, i)
 			((u64 *)__get_dynamic_array(phys_addrs))[i] = sg_phys(sg);
-		for_each_sg(sgl, sg, ents, i) {
+		for_each_sg(sgl, sg, traced_ents, i) {
 			((u64 *)__get_dynamic_array(dma_addrs))[i] =
 				sg_dma_address(sg);
 			((unsigned int *)__get_dynamic_array(lengths))[i] =
-- 
2.43.0