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>
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9a2fa14720
28
fs/file.c
28
fs/file.c
@ -46,27 +46,23 @@ static void free_fdtable_rcu(struct rcu_head *rcu)
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#define BITBIT_NR(nr) BITS_TO_LONGS(BITS_TO_LONGS(nr))
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#define BITBIT_SIZE(nr) (BITBIT_NR(nr) * sizeof(long))
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#define fdt_words(fdt) ((fdt)->max_fds / BITS_PER_LONG) // words in ->open_fds
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/*
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* Copy 'count' fd bits from the old table to the new table and clear the extra
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* space if any. This does not copy the file pointers. Called with the files
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* spinlock held for write.
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*/
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static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
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unsigned int count)
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static inline void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
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unsigned int copy_words)
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{
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unsigned int cpy, set;
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unsigned int nwords = fdt_words(nfdt);
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cpy = count / BITS_PER_BYTE;
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set = (nfdt->max_fds - count) / BITS_PER_BYTE;
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memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
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memset((char *)nfdt->open_fds + cpy, 0, set);
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memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
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memset((char *)nfdt->close_on_exec + cpy, 0, set);
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cpy = BITBIT_SIZE(count);
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set = BITBIT_SIZE(nfdt->max_fds) - cpy;
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memcpy(nfdt->full_fds_bits, ofdt->full_fds_bits, cpy);
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memset((char *)nfdt->full_fds_bits + cpy, 0, set);
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bitmap_copy_and_extend(nfdt->open_fds, ofdt->open_fds,
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copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
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bitmap_copy_and_extend(nfdt->close_on_exec, ofdt->close_on_exec,
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copy_words * BITS_PER_LONG, nwords * BITS_PER_LONG);
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bitmap_copy_and_extend(nfdt->full_fds_bits, ofdt->full_fds_bits,
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copy_words, nwords);
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}
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/*
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@ -84,7 +80,7 @@ static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
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memcpy(nfdt->fd, ofdt->fd, cpy);
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memset((char *)nfdt->fd + cpy, 0, set);
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copy_fd_bitmaps(nfdt, ofdt, ofdt->max_fds);
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copy_fd_bitmaps(nfdt, ofdt, fdt_words(ofdt));
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}
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/*
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@ -379,7 +375,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, unsigned int max_fds, int
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open_files = sane_fdtable_size(old_fdt, max_fds);
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}
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copy_fd_bitmaps(new_fdt, old_fdt, open_files);
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copy_fd_bitmaps(new_fdt, old_fdt, open_files / BITS_PER_LONG);
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old_fds = old_fdt->fd;
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new_fds = new_fdt->fd;
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@ -270,6 +270,18 @@ static inline void bitmap_copy_clear_tail(unsigned long *dst,
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dst[nbits / BITS_PER_LONG] &= BITMAP_LAST_WORD_MASK(nbits);
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}
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static inline void bitmap_copy_and_extend(unsigned long *to,
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const unsigned long *from,
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unsigned int count, unsigned int size)
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{
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unsigned int copy = BITS_TO_LONGS(count);
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memcpy(to, from, copy * sizeof(long));
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if (count % BITS_PER_LONG)
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to[copy - 1] &= BITMAP_LAST_WORD_MASK(count);
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memset(to + copy, 0, bitmap_size(size) - copy * sizeof(long));
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}
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/*
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* On 32-bit systems bitmaps are represented as u32 arrays internally. On LE64
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* machines the order of hi and lo parts of numbers match the bitmap structure.
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@ -589,4 +589,39 @@ TEST(close_range_cloexec_unshare_syzbot)
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EXPECT_EQ(close(fd3), 0);
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}
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TEST(close_range_bitmap_corruption)
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{
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pid_t pid;
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int status;
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struct __clone_args args = {
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.flags = CLONE_FILES,
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.exit_signal = SIGCHLD,
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};
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/* get the first 128 descriptors open */
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for (int i = 2; i < 128; i++)
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EXPECT_GE(dup2(0, i), 0);
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/* get descriptor table shared */
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pid = sys_clone3(&args, sizeof(args));
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ASSERT_GE(pid, 0);
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if (pid == 0) {
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/* unshare and truncate descriptor table down to 64 */
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if (sys_close_range(64, ~0U, CLOSE_RANGE_UNSHARE))
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exit(EXIT_FAILURE);
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ASSERT_EQ(fcntl(64, F_GETFD), -1);
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/* ... and verify that the range 64..127 is not
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stuck "fully used" according to secondary bitmap */
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EXPECT_EQ(dup(0), 64)
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exit(EXIT_FAILURE);
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exit(EXIT_SUCCESS);
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}
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EXPECT_EQ(waitpid(pid, &status, 0), pid);
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EXPECT_EQ(true, WIFEXITED(status));
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EXPECT_EQ(0, WEXITSTATUS(status));
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}
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TEST_HARNESS_MAIN
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