fix bitmap corruption on close_range() with CLOSE_RANGE_UNSHARE
commit 9a2fa1472083580b6c66bdaf291f591e1170123a upstream.
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
BUG=b/366703659
TEST=presubmit
RELEASE_NOTE=Fixes CVE-2024-45025 in the Linux kernel
cos-patch: security-moderate
Cc: stable@vger.kernel.org
Change-Id: I373bfaa387e58abf446a78fc0a27acda477bb41f
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kernel CVE Triage Automation <cloud-image-kernel-cve-triage-automation@prod.google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/81144
Reviewed-by: Arnav Kansal <rnv@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Kevin Berry <kpberry@google.com>
diff --git a/fs/file.c b/fs/file.c
index dbca26e..0476fd3 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;
+ unsigned int nwords = fdt_words(nfdt);
- 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);
+ 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));
}
/*
@@ -374,7 +370,7 @@ 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 63e422f..3c8d2b8 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -281,6 +281,18 @@ 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 7492399..190c57b 100644
--- a/tools/testing/selftests/core/close_range_test.c
+++ b/tools/testing/selftests/core/close_range_test.c
@@ -563,4 +563,39 @@ 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
