[U-Boot] [PATCH v2 8/8] tftp: prevent overwriting reserved memory

[Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>]

This fixes CVE-2018-18439 ("insufficient boundary checks in network
image boot") by using lmb to check for a valid range to store
received blocks.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
---

Changes in v2:
- this patch is new in v2

 net/tftp.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 57 insertions(+), 9 deletions(-)

diff --git a/net/tftp.c b/net/tftp.c
index 563ce3a06f..390394199d 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -17,6 +17,8 @@
 #include <flash.h>
 #endif
 
+DECLARE_GLOBAL_DATA_PTR;
+
 /* Well known TFTP port # */
 #define WELL_KNOWN_PORT	69
 /* Millisecs to timeout for lost pkt */
@@ -81,6 +83,8 @@ static ulong	tftp_block_wrap;
 /* memory offset due to wrapping */
 static ulong	tftp_block_wrap_offset;
 static int	tftp_state;
+static ulong	tftp_load_addr;
+static ulong	tftp_load_size;
 #ifdef CONFIG_TFTP_TSIZE
 /* The file size reported by the server */
 static int	tftp_tsize;
@@ -134,10 +138,11 @@ static char tftp_filename[MAX_LEN];
 static unsigned short tftp_block_size = TFTP_BLOCK_SIZE;
 static unsigned short tftp_block_size_option = TFTP_MTU_BLOCKSIZE;
 
-static inline void store_block(int block, uchar *src, unsigned len)
+static inline int store_block(int block, uchar *src, unsigned int len)
 {
 	ulong offset = block * tftp_block_size + tftp_block_wrap_offset;
 	ulong newsize = offset + len;
+	ulong store_addr = tftp_load_addr + offset;
 #ifdef CONFIG_SYS_DIRECT_FLASH_TFTP
 	int i, rc = 0;
 
@@ -145,30 +150,38 @@ static inline void store_block(int block, uchar *src, unsigned len)
 		/* start address in flash? */
 		if (flash_info[i].flash_id == FLASH_UNKNOWN)
 			continue;
-		if (load_addr + offset >= flash_info[i].start[0]) {
+		if (store_addr >= flash_info[i].start[0]) {
 			rc = 1;
 			break;
 		}
 	}
 
 	if (rc) { /* Flash is destination for this packet */
-		rc = flash_write((char *)src, (ulong)(load_addr+offset), len);
+		rc = flash_write((char *)src, store_addr, len);
 		if (rc) {
 			flash_perror(rc);
-			net_set_state(NETLOOP_FAIL);
-			return;
+			return rc;
 		}
 	} else
 #endif /* CONFIG_SYS_DIRECT_FLASH_TFTP */
 	{
-		void *ptr = map_sysmem(load_addr + offset, len);
+		void *ptr;
 
+		if (store_addr < tftp_load_addr ||
+		    store_addr + len > tftp_load_addr + tftp_load_size) {
+			puts("\nTFTP error: ");
+			puts("trying to overwrite reserved memory...\n");
+			return -1;
+		}
+		ptr = map_sysmem(store_addr, len);
 		memcpy(ptr, src, len);
 		unmap_sysmem(ptr);
 	}
 
 	if (net_boot_file_size < newsize)
 		net_boot_file_size = newsize;
+
+	return 0;
 }
 
 /* Clear our state ready for a new transfer */
@@ -527,7 +540,11 @@ static void tftp_handler(uchar *pkt, unsigned dest, struct in_addr sip,
 		timeout_count_max = tftp_timeout_count_max;
 		net_set_timeout_handler(timeout_ms, tftp_timeout_handler);
 
-		store_block(tftp_cur_block - 1, pkt + 2, len);
+		if (store_block(tftp_cur_block - 1, pkt + 2, len)) {
+			eth_halt();
+			net_set_state(NETLOOP_FAIL);
+			break;
+		}
 
 		/*
 		 *	Acknowledge the block just received, which will prompt
@@ -577,6 +594,24 @@ static void tftp_timeout_handler(void)
 	}
 }
 
+/* Initialize tftp_load_addr and tftp_load_size from load_addr and lmb */
+static int tftp_init_load_addr(void)
+{
+	struct lmb lmb;
+	phys_size_t max_size;
+
+	tftp_load_addr = load_addr;
+
+	lmb_init_and_reserve(&lmb, gd->bd->bi_dram[0].start,
+			     gd->bd->bi_dram[0].size, (void *)gd->fdt_blob);
+
+	max_size = lmb_get_unreserved_size(&lmb, tftp_load_addr);
+	if (!max_size)
+		return -1;
+
+	tftp_load_size = max_size;
+	return 0;
+}
 
 void tftp_start(enum proto_t protocol)
 {
@@ -673,7 +708,14 @@ void tftp_start(enum proto_t protocol)
 	} else
 #endif
 	{
-		printf("Load address: 0x%lx\n", load_addr);
+		if (tftp_init_load_addr()) {
+			eth_halt();
+			net_set_state(NETLOOP_FAIL);
+			puts("\nTFTP error: ");
+			puts("trying to overwrite reserved memory...\n");
+			return;
+		}
+		printf("Load address: 0x%lx\n", tftp_load_addr);
 		puts("Loading: *\b");
 		tftp_state = STATE_SEND_RRQ;
 #ifdef CONFIG_CMD_BOOTEFI
@@ -721,9 +763,15 @@ void tftp_start_server(void)
 {
 	tftp_filename[0] = 0;
 
+	if (tftp_init_load_addr()) {
+		eth_halt();
+		net_set_state(NETLOOP_FAIL);
+		puts("\nTFTP error: trying to overwrite reserved memory...\n");
+		return;
+	}
 	printf("Using %s device\n", eth_get_name());
 	printf("Listening for TFTP transfer on %pI4\n", &net_ip);
-	printf("Load address: 0x%lx\n", load_addr);
+	printf("Load address: 0x%lx\n", tftp_load_addr);
 
 	puts("Loading: *\b");
 
-- 
2.17.1