Change in coreboot[master]: libpayload: cbfs: fix infinite loop in cbfs_get_{handle,attr}

[Alexandre Rebert (Code Review)]

Alexandre Rebert has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/39177 )

Change subject: libpayload: cbfs: fix infinite loop in cbfs_get_{handle,attr}
......................................................................

libpayload: cbfs: fix infinite loop in cbfs_get_{handle,attr}

cbfs_get_handle() and cbfs_get_attr() are both looping over elements to
find a particular one. Each element header contains the element's
length, which is used to compute the next element's offset. Invalid or
corrupted CBFS files could lead to infinite loops where the offset would
remain constant across iterations, due to 0-length elements or integer
overflows in the computation of the next offset.

This patch makes both functions more robust by adding a check that
ensure offsets are strictly monotonic. Instead of infinite looping, the
functions are now printing an ERROR and returning a NULL value.

Change-Id: I440e82fa969b8c2aacc5800e7e26450c3b97c74a
Signed-off-by: Alex Rebert 
Found-by: Mayhem
---
M payloads/libpayload/libcbfs/cbfs_core.c
1 file changed, 17 insertions(+), 4 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/77/39177/1

diff --git a/payloads/libpayload/libcbfs/cbfs_core.c b/payloads/libpayload/libcbfs/cbfs_core.c
index e94e1e7..54e5de8 100644
--- a/payloads/libpayload/libcbfs/cbfs_core.c
+++ b/payloads/libpayload/libcbfs/cbfs_core.c
@@ -212,9 +212,15 @@
    	}

// Move to next file.
-		offset += ntohl(file.len) + ntohl(file.offset);
-		if (offset % CBFS_ALIGNMENT)
-			offset += CBFS_ALIGNMENT - (offset % CBFS_ALIGNMENT);
+		uint32_t new_offset = offset + ntohl(file.len) + ntohl(file.offset);
+		if (new_offset % CBFS_ALIGNMENT)
+			new_offset += CBFS_ALIGNMENT - (new_offset % CBFS_ALIGNMENT);
+		// Check that offset is strictly monotonic to prevent infinite loop
+		if(new_offset <= offset) {
+			ERROR("ERROR: corrupted CBFS file header at 0x%x.\n", offset);
+			break;
+		}
+		offset = new_offset;
    }
    media->close(media);
    LOG("WARNING: '%s' not found.\n", name);
@@ -309,7 +315,14 @@
    		return NULL;
    	}
    	if (ntohl(attr.tag) != tag) {
-			offset += ntohl(attr.len);
+			uint32_t new_offset = offset + ntohl(attr.len);
+			// Check that offset is strictly monotonic to prevent infinite loop
+			if(new_offset <= offset) {
+				ERROR("ERROR: corrupted CBFS attribute at 0x%x.\n", offset);
+				m->close(m);
+				return NULL;
+			}
+			offset = new_offset;
    		continue;
    	}
    	ret = m->map(m, offset, ntohl(attr.len));