Change in flashrom[master]: linux_mtd: prevent corruption of flash when stdout/stderr is closed

[Jack Rosenthal (Code Review)]

Jack Rosenthal has uploaded this change for review. ( https://review.coreboot.org/c/flashrom/+/52215 )

Change subject: linux_mtd: prevent corruption of flash when stdout/stderr is closed
......................................................................

linux_mtd: prevent corruption of flash when stdout/stderr is closed

While it's not posixly-correct, it's possible that a user, script, or
application may attempt to start flashrom with stdout or stderr
closed.  If this happens with an mtd device, it's possible that we'll
get a file descriptor of 1 or 2, and flashrom will send garbage debug
logs to the flash:

# bash -c "exec >&- flashrom ..."

Observed corruption:
  43 40 45 42 45 44 00 00 00 00 00 00 01 00 00 00 |C@EBED..........|
  00 02 00 00 63 65 73 73 66 75 6c 6c 79 0a 46 6f |....cessfully.Fo|
  75 6e 64 20 50 72 6f 67 72 61 6d 6d 65 72 20 66 |und Programmer f|
  6c 61 73 68 20 63 68 69 70 20 22 4f 70 61 71 75 |lash chip "Opaqu|
  65 20 66 6c 61 73 68 20 63 68 69 70 22 20 28 38 |e flash chip" (8|
  31 39 32 20 6b 42 2c 20 50 72 6f 67 72 61 6d 6d |192 kB, Programm|
  65 72 2d 73 70 65 63 69 66 69 63 29 20 6d 61 70 |er-specific) map|
  70 65 64 20 61 74 20 70 68 79 73 69 63 61 6c 20 |ped at physical |
  61 64 64 72 65 73 73 20 30 78 30 30 30 30 30 30 |address 0x000000|
  30 30 2e 0a ff ff ff ff ff ff ff ff ff ff ff ff |00..............|
  ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
  ...

While for most applications, closing stdout or stderr would just lead
to obsure bugs, for flashrom, we should have extra safety guards, as
this could mean that we might be bricking a device instead.

Add a basic safety check.

Signed-off-by: Jack Rosenthal 
Change-Id: I751c9dd88ad1d30283b94bd2185b4f8f25569c8f
---
M linux_mtd.c
1 file changed, 20 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/flashrom refs/changes/15/52215/1

diff --git a/linux_mtd.c b/linux_mtd.c
index 22702e9..aa4fe4f 100644
--- a/linux_mtd.c
+++ b/linux_mtd.c
@@ -340,6 +340,26 @@
    	msg_perr("Cannot open file stream for %s\n", dev_path);
    	goto linux_mtd_setup_exit;
    }
+
+	/*
+	 * Safety guard against accidentally writing stdout or stderr
+	 * to the flash.  In case flashrom was started with stdout or
+	 * stderr closed, it's possible that we will get fd 1 or fd 2
+	 * for the flash device.  We don't want to accidentally write
+	 * debug messages to the flash device, corrupting the
+	 * flash.
+	 */
+	if (fileno(dev_fp) <= 2) {
+		fclose(dev_fp);
+		/*
+		 * This message may not actually make it to the user,
+		 * given stdout/stderr may be closed.  But we can at
+		 * least try.
+		 */
+		msg_perr("stdout or stderr is closed\n");
+		goto linux_mtd_setup_exit;
+	}
+
    ret = setvbuf(dev_fp, NULL, _IONBF, 0);
    if (ret)
    	msg_pwarn("Failed to set MTD device to unbuffered: %d\n", ret);