fix bitmap corruption on close_range() with CLOSE_RANGE_UNSHARE

copy_fd_bitmaps(new, old, count) is expected to copy the first
count/BITS_PER_LONG bits from old->full_fds_bits[] and fill
the rest with zeroes.  What it does is copying enough words
(BITS_TO_LONGS(count/BITS_PER_LONG)), then memsets the rest.
That works fine, *if* all bits past the cutoff point are
clear.  Otherwise we are risking garbage from the last word
we'd copied.

For most of the callers that is true - expand_fdtable() has
count equal to old->max_fds, so there's no open descriptors
past count, let alone fully occupied words in ->open_fds[],
which is what bits in ->full_fds_bits[] correspond to.

The other caller (dup_fd()) passes sane_fdtable_size(old_fdt, max_fds),
which is the smallest multiple of BITS_PER_LONG that covers all
opened descriptors below max_fds.  In the common case (copying on
fork()) max_fds is ~0U, so all opened descriptors will be below
it and we are fine, by the same reasons why the call in expand_fdtable()
is safe.

Unfortunately, there is a case where max_fds is less than that
and where we might, indeed, end up with junk in ->full_fds_bits[] -
close_range(from, to, CLOSE_RANGE_UNSHARE) with
	* descriptor table being currently shared
	* 'to' being above the current capacity of descriptor table
	* 'from' being just under some chunk of opened descriptors.
In that case we end up with observably wrong behaviour - e.g. spawn
a child with CLONE_FILES, get all descriptors in range 0..127 open,
then close_range(64, ~0U, CLOSE_RANGE_UNSHARE) and watch dup(0) ending
up with descriptor #128, despite #64 being observably not open.

The minimally invasive fix would be to deal with that in dup_fd().
If this proves to add measurable overhead, we can go that way, but
let's try to fix copy_fd_bitmaps() first.

* new helper: bitmap_copy_and_expand(to, from, bits_to_copy, size).
* make copy_fd_bitmaps() take the bitmap size in words, rather than
bits; it's 'count' argument is always a multiple of BITS_PER_LONG,
so we are not losing any information, and that way we can use the
same helper for all three bitmaps - compiler will see that count
is a multiple of BITS_PER_LONG for the large ones, so it'll generate
plain memcpy()+memset().

Reproducer added to tools/testing/selftests/core/close_range_test.c

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 35 insertions, 0 deletions

diff --git a/tools/testing/selftests/core/close_range_test.c b/tools/testing/selftests/core/close_range_test.c
index 991c473e3859..12b4eb9d0434 100644
--- a/tools/testing/selftests/core/close_range_test.c
+++ b/tools/testing/selftests/core/close_range_test.c
@@ -589,4 +589,39 @@ TEST(close_range_cloexec_unshare_syzbot)
 	EXPECT_EQ(close(fd3), 0);
 }
 
+TEST(close_range_bitmap_corruption)
+{
+	pid_t pid;
+	int status;
+	struct __clone_args args = {
+		.flags = CLONE_FILES,
+		.exit_signal = SIGCHLD,
+	};
+
+	/* get the first 128 descriptors open */
+	for (int i = 2; i < 128; i++)
+		EXPECT_GE(dup2(0, i), 0);
+
+	/* get descriptor table shared */
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* unshare and truncate descriptor table down to 64 */
+		if (sys_close_range(64, ~0U, CLOSE_RANGE_UNSHARE))
+			exit(EXIT_FAILURE);
+
+		ASSERT_EQ(fcntl(64, F_GETFD), -1);
+		/* ... and verify that the range 64..127 is not
+		   stuck "fully used" according to secondary bitmap */
+		EXPECT_EQ(dup(0), 64)
+			exit(EXIT_FAILURE);
+		exit(EXIT_SUCCESS);
+	}
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
 TEST_HARNESS_MAIN