Re: [PATCH] fix bitmap corruption on close_range() with CLOSE_RANGE_UNSHARE

Al Viro
2024-08-05T18:54:31Z

On Mon, Aug 05, 2024 at 09:22:08AM +0200, Christian Brauner wrote:

> Really, it doesn't have to be pretty but these repros in there really
> have been helpful finding such corruptions when run with a proper k*san
> config.

See below; so far it survives beating (and close_range_test passes with
the patch, while failing the last test on mainline). BTW, EXPECT_...
alone is sufficient for it to whine, but it doesn't actually fail the
test, so don't we need exit(EXIT_FAILURE) on (at least some of) the
previous tests? Hadn't played with the kselftest before, so...

diff --git a/fs/file.c b/fs/file.c
index a11e59b5d602..655338effe9c 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -46,27 +46,23 @@ static void free_fdtable_rcu(struct rcu_head *rcu)
#define BITBIT_NR(nr) BITS_TO_LONGS(BITS_TO_LONGS(nr))
#define BITBIT_SIZE(nr) (BITBIT_NR(nr) * sizeof(long))

+#define fdt_words(fdt) ((fdt)->max_fds / BITS_PER_LONG) // words in ->open_fds
/*
* Copy 'count' fd bits from the old table to the new table and clear the extra
* space if any. This does not copy the file pointers. Called with the files
* spinlock held for write.
*/
-static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
- unsigned int count)
+static inline void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
+ unsigned int copy_words)
{
- unsigned int cpy, set;
-
- cpy = count / BITS_PER_BYTE;
- set = (nfdt->max_fds - count) / BITS_PER_BYTE;
- memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
- memset((char *)nfdt->open_fds + cpy, 0, set);
- memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
- memset((char *)nfdt->close_on_exec + cpy, 0, set);
-
- cpy = BITBIT_SIZE(count);
- set = BITBIT_SIZE(nfdt->max_fds) - cpy;
- memcpy(nfdt->full_fds_bits, ofdt->full_fds_bits, cpy);
- memset((char *)nfdt->full_fds_bits + cpy, 0, set);
+ unsigned int nwords = fdt_words(nfdt);
+
+ bitmap_copy_and_extend(nfdt->open_fds, ofdt->open_fds,
+ copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
+ bitmap_copy_and_extend(nfdt->close_on_exec, ofdt->close_on_exec,
+ copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
+ bitmap_copy_and_extend(nfdt->full_fds_bits, ofdt->full_fds_bits,
+ copy_words, nwords);
}

/*
@@ -84,7 +80,7 @@ static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
memcpy(nfdt->fd, ofdt->fd, cpy);
memset((char *)nfdt->fd + cpy, 0, set);

- copy_fd_bitmaps(nfdt, ofdt, ofdt->max_fds);
+ copy_fd_bitmaps(nfdt, ofdt, fdt_words(ofdt));
}

/*
@@ -379,7 +375,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, unsigned int max_fds, int
open_files = sane_fdtable_size(old_fdt, max_fds);
}

- copy_fd_bitmaps(new_fdt, old_fdt, open_files);
+ copy_fd_bitmaps(new_fdt, old_fdt, open_files / BITS_PER_LONG);

old_fds = old_fdt->fd;
new_fds = new_fdt->fd;
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 8c4768c44a01..d3b66d77df7a 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -270,6 +270,18 @@ static inline void bitmap_copy_clear_tail(unsigned long *dst,
dst[nbits / BITS_PER_LONG] &= BITMAP_LAST_WORD_MASK(nbits);
}

+static inline void bitmap_copy_and_extend(unsigned long *to,
+ const unsigned long *from,
+ unsigned int count, unsigned int size)
+{
+ unsigned int copy = BITS_TO_LONGS(count);
+
+ memcpy(to, from, copy * sizeof(long));
+ if (count % BITS_PER_LONG)
+ to[copy - 1] &= BITMAP_LAST_WORD_MASK(count);
+ memset(to + copy, 0, bitmap_size(size) - copy * sizeof(long));
+}
+
/*
* On 32-bit systems bitmaps are represented as u32 arrays internally. On LE64
* machines the order of hi and lo parts of numbers match the bitmap structure.
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