cache: close race window when unlocking slots We use POSIX advisory record locks to control access to cache slots, but these have an unhelpful behaviour in that they are released when any file descriptor referencing the file is closed by this process. Mostly this is okay, since we know we won't be opening the lock file anywhere else, but there is one place that it does matter: when we restore stdout we dup2() over a file descriptor referring to the file, thus closing that descriptor. Since we restore stdout before unlocking the slot, this creates a window during which the slot content can be overwritten. The fix is reasonably straightforward: simply restore stdout after unlocking the slot, but the diff is a bit bigger because this requires us to move the temporary stdout FD into struct cache_slot. Signed-off-by: John Keeping <john@keeping.me.uk> Reviewed-by: Christian Hesse <mail@eworm.de>
John Keeping john@keeping.me.uk
Wed, 20 Jun 2018 07:29:14 +0200
1 files changed,
14 insertions(+),
23 deletions(-)
jump to
M
cache.c
→
cache.c
@@ -29,6 +29,7 @@ int ttl;
cache_fill_fn fn; int cache_fd; int lock_fd; + int stdout_fd; const char *cache_name; const char *lock_name; int match;@@ -197,6 +198,13 @@ err = rename(slot->lock_name, slot->cache_name);
else err = unlink(slot->lock_name); + /* Restore stdout and close the temporary FD. */ + if (slot->stdout_fd >= 0) { + dup2(slot->stdout_fd, STDOUT_FILENO); + close(slot->stdout_fd); + slot->stdout_fd = -1; + } + if (err) return errno;@@ -208,42 +216,24 @@ * stdout to the lock-fd and invoking the callback function
*/ static int fill_slot(struct cache_slot *slot) { - int tmp; - /* Preserve stdout */ - tmp = dup(STDOUT_FILENO); - if (tmp == -1) + slot->stdout_fd = dup(STDOUT_FILENO); + if (slot->stdout_fd == -1) return errno; /* Redirect stdout to lockfile */ - if (dup2(slot->lock_fd, STDOUT_FILENO) == -1) { - close(tmp); + if (dup2(slot->lock_fd, STDOUT_FILENO) == -1) return errno; - } /* Generate cache content */ slot->fn(); /* Make sure any buffered data is flushed to the file */ - if (fflush(stdout)) { - close(tmp); + if (fflush(stdout)) return errno; - } /* update stat info */ - if (fstat(slot->lock_fd, &slot->cache_st)) { - close(tmp); - return errno; - } - - /* Restore stdout */ - if (dup2(tmp, STDOUT_FILENO) == -1) { - close(tmp); - return errno; - } - - /* Close the temporary filedescriptor */ - if (close(tmp)) + if (fstat(slot->lock_fd, &slot->cache_st)) return errno; return 0;@@ -393,6 +383,7 @@ strbuf_addbuf(&lockname, &filename);
strbuf_addstr(&lockname, ".lock"); slot.fn = fn; slot.ttl = ttl; + slot.stdout_fd = -1; slot.cache_name = filename.buf; slot.lock_name = lockname.buf; slot.key = key;