Re: [PATCH v5 06/24] fsverity: pass tree_blocksize to end_enable_verity()

David Hildenbrand
2024-03-13T19:10:46Z

On 13.03.24 18:19, Darrick J. Wong wrote:
> On Wed, Mar 13, 2024 at 01:29:12PM +0100, David Hildenbrand wrote:
>> On 12.03.24 17:44, Darrick J. Wong wrote:
>>> On Tue, Mar 12, 2024 at 04:33:14PM +0100, David Hildenbrand wrote:
>>>> On 12.03.24 16:13, David Hildenbrand wrote:
>>>>> On 11.03.24 23:38, Darrick J. Wong wrote:
>>>>>> [add willy and linux-mm]
>>>>>>
>>>>>> On Thu, Mar 07, 2024 at 08:40:17PM -0800, Eric Biggers wrote:
>>>>>>> On Thu, Mar 07, 2024 at 07:46:50PM -0800, Darrick J. Wong wrote:
>>>>>>>>> BTW, is xfs_repair planned to do anything about any such extra blocks?
>>>>>>>>
>>>>>>>> Sorry to answer your question with a question, but how much checking is
>>>>>>>> $filesystem expected to do for merkle trees?
>>>>>>>>
>>>>>>>> In theory xfs_repair could learn how to interpret the verity descriptor,
>>>>>>>> walk the merkle tree blocks, and even read the file data to confirm
>>>>>>>> intactness. If the descriptor specifies the highest block address then
>>>>>>>> we could certainly trim off excess blocks. But I don't know how much of
>>>>>>>> libfsverity actually lets you do that; I haven't looked into that
>>>>>>>> deeply. :/
>>>>>>>>
>>>>>>>> For xfs_scrub I guess the job is theoretically simpler, since we only
>>>>>>>> need to stream reads of the verity files through the page cache and let
>>>>>>>> verity tell us if the file data are consistent.
>>>>>>>>
>>>>>>>> For both tools, if something finds errors in the merkle tree structure
>>>>>>>> itself, do we turn off verity? Or do we do something nasty like
>>>>>>>> truncate the file?
>>>>>>>
>>>>>>> As far as I know (I haven't been following btrfs-progs, but I'm familiar with
>>>>>>> e2fsprogs and f2fs-tools), there isn't yet any precedent for fsck actually
>>>>>>> validating the data of verity inodes against their Merkle trees.
>>>>>>>
>>>>>>> e2fsck does delete the verity metadata of inodes that don't have the verity flag
>>>>>>> enabled. That handles cleaning up after a crash during FS_IOC_ENABLE_VERITY.
>>>>>>>
>>>>>>> I suppose that ideally, if an inode's verity metadata is invalid, then fsck
>>>>>>> should delete that inode's verity metadata and remove the verity flag from the
>>>>>>> inode. Checking for a missing or obviously corrupt fsverity_descriptor would be
>>>>>>> fairly straightforward, but it probably wouldn't catch much compared to actually
>>>>>>> validating the data against the Merkle tree. And actually validating the data
>>>>>>> against the Merkle tree would be complex and expensive. Note, none of this
>>>>>>> would work on files that are encrypted.
>>>>>>>
>>>>>>> Re: libfsverity, I think it would be possible to validate a Merkle tree using
>>>>>>> libfsverity_compute_digest() and the callbacks that it supports. But that's not
>>>>>>> quite what it was designed for.
>>>>>>>
>>>>>>>> Is there an ioctl or something that allows userspace to validate an
>>>>>>>> entire file's contents? Sort of like what BLKVERIFY would have done for
>>>>>>>> block devices, except that we might believe its answers?
>>>>>>>
>>>>>>> Just reading the whole file and seeing whether you get an error would do it.
>>>>>>>
>>>>>>> Though if you want to make sure it's really re-reading the on-disk data, it's
>>>>>>> necessary to drop the file's pagecache first.
>>>>>>
>>>>>> I tried a straight pagecache read and it worked like a charm!
>>>>>>
>>>>>> But then I thought to myself, do I really want to waste memory bandwidth
>>>>>> copying a bunch of data? No. I don't even want to incur system call
>>>>>> overhead from reading a single byte every $pagesize bytes.
>>>>>>
>>>>>> So I created 2M mmap areas and read a byte every $pagesize bytes. That
>>>>>> worked too, insofar as SIGBUSes are annoying to handle. But it's
>>>>>> annoying to take signals like that.
>>>>>>
>>>>>> Then I started looking at madvise. MADV_POPULATE_READ looked exactly
>>>>>> like what I wanted -- it prefaults in the pages, and "If populating
>>>>>> fails, a SIGBUS signal is not generated; instead, an error is returned."
>>>>>>
>>>>>
>>>>> Yes, these were the expected semantics :)
>>>>>
>>>>>> But then I tried rigging up a test to see if I could catch an EIO, and
>>>>>> instead I had to SIGKILL the process! It looks filemap_fault returns
>>>>>> VM_FAULT_RETRY to __xfs_filemap_fault, which propagates up through
>>>>>> __do_fault -> do_read_fault -> do_fault -> handle_pte_fault ->
>>>>>> handle_mm_fault -> faultin_page -> __get_user_pages. At faultin_pages,
>>>>>> the VM_FAULT_RETRY is translated to -EBUSY.
>>>>>>
>>>>>> __get_user_pages squashes -EBUSY to 0, so faultin_vma_page_range returns
>>>>>> that to madvise_populate. Unfortunately, madvise_populate increments
>>>>>> its loop counter by the return value (still 0) so it runs in an
>>>>>> infinite loop. The only way out is SIGKILL.
>>>>>
>>>>> That's certainly unexpected. One user I know is QEMU, which primarily
>>>>> uses MADV_POPULATE_WRITE to prefault page tables. Prefaulting in QEMU is
>>>>> primarily used with shmem/hugetlb, where I haven't heard of any such
>>>>> endless loops.
>>>>>
>>>>>>
>>>>>> So I don't know what the correct behavior is here, other than the
>>>>>> infinite loop seems pretty suspect. Is it the correct behavior that
>>>>>> madvise_populate returns EIO if __get_user_pages ever returns zero?
>>>>>> That doesn't quite sound right if it's the case that a zero return could
>>>>>> also happen if memory is tight.
>>>>>
>>>>> madvise_populate() ends up calling faultin_vma_page_range() in a loop.
>>>>> That one calls __get_user_pages().
>>>>>
>>>>> __get_user_pages() documents: "0 return value is possible when the fault
>>>>> would need to be retried."
>>>>>
>>>>> So that's what the caller does. IIRC, there are cases where we really
>>>>> have to retry (at least once) and will make progress, so treating "0" as
>>>>> an error would be wrong.
>>>>>
>>>>> Staring at other __get_user_pages() users, __get_user_pages_locked()
>>>>> documents: "Please note that this function, unlike __get_user_pages(),
>>>>> will not return 0 for nr_pages > 0, unless FOLL_NOWAIT is used.".
>>>>>
>>>>> But there is some elaborate retry logic in there, whereby the retry will
>>>>> set FOLL_TRIED->FAULT_FLAG_TRIED, and I think we'd fail on the second
>>>>> retry attempt (there are cases where we retry more often, but that's
>>>>> related to something else I believe).
>>>>>
>>>>> So maybe we need a similar retry logic in faultin_vma_page_range()? Or
>>>>> make it use __get_user_pages_locked(), but I recall when I introduced
>>>>> MADV_POPULATE_READ, there was a catch to it.
>>>>
>>>> I'm trying to figure out who will be setting the VM_FAULT_SIGBUS in the
>>>> mmap()+access case you describe above.
>>>>
>>>> Staring at arch/x86/mm/fault.c:do_user_addr_fault(), I don't immediately see
>>>> how we would transition from a VM_FAULT_RETRY loop to VM_FAULT_SIGBUS.
>>>> Because VM_FAULT_SIGBUS would be required for that function to call
>>>> do_sigbus().
>>>
>>> The code I was looking at yesterday in filemap_fault was:
>>>
>>> page_not_uptodate:
>>> /*
>>> * Umm, take care of errors if the page isn't up-to-date.
>>> * Try to re-read it _once_. We do this synchronously,
>>> * because there really aren't any performance issues here
>>> * and we need to check for errors.
>>> */
>>> fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>>> error = filemap_read_folio(file, mapping->a_ops->read_folio, folio);
>>> if (fpin)
>>> goto out_retry;
>>> folio_put(folio);
>>>
>>> if (!error || error == AOP_TRUNCATED_PAGE)
>>> goto retry_find;
>>> filemap_invalidate_unlock_shared(mapping);
>>>
>>> return VM_FAULT_SIGBUS;
>>>
>>> Wherein I /think/ fpin is non-null in this case, so if
>>> filemap_read_folio returns an error, we'll do this instead:
>>>
>>> out_retry:
>>> /*
>>> * We dropped the mmap_lock, we need to return to the fault handler to
>>> * re-find the vma and come back and find our hopefully still populated
>>> * page.
>>> */
>>> if (!IS_ERR(folio))
>>> folio_put(folio);
>>> if (mapping_locked)
>>> filemap_invalidate_unlock_shared(mapping);
>>> if (fpin)
>>> fput(fpin);
>>> return ret | VM_FAULT_RETRY;
>>>
>>> and since ret was 0 before the goto, the only return code is
>>> VM_FAULT_RETRY. I had speculated that perhaps we could instead do:
>>>
>>> if (fpin) {
>>> if (error)
>>> ret |= VM_FAULT_SIGBUS;
>>> goto out_retry;
>>> }
>>>
>>> But I think the hard part here is that there doesn't seem to be any
>>> distinction between transient read errors (e.g. disk cable fell out) vs.
>>> semi-permanent errors (e.g. verity says the hash doesn't match).
>>> AFAICT, either the read(ahead) sets uptodate and callers read the page,
>>> or it doesn't set it and callers treat that as an error-retry
>>> opportunity.
>>>
>>> For the transient error case VM_FAULT_RETRY makes perfect sense; for the
>>> second case I imagine we'd want something closer to _SIGBUS.
>>
>>
>> Agreed, it's really hard to judge when it's the right time to give up
>> retrying. At least with MADV_POPULATE_READ we should try achieving the same
>> behavior as with mmap()+read access. So if the latter manages to trigger
>> SIGBUS, MADV_POPULATE_READ should return an error.
>>
>> Is there an easy way to for me to reproduce this scenario?
>
> Yes. Take this Makefile:
>
> CFLAGS=-Wall -Werror -O2 -g -Wno-unused-variable
>
> all: mpr
>
> and this C program mpr.c:
>
> /* test MAP_POPULATE_READ on a file */
> #include <stdio.h>
> #include <errno.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <string.h>
> #include <sys/stat.h>
> #include <sys/mman.h>
>
> #define min(a, b) ((a) < (b) ? (a) : (b))
> #define BUFSIZE (2097152)
>
> int main(int argc, char *argv[])
> {
> struct stat sb;
> long pagesize = sysconf(_SC_PAGESIZE);
> off_t read_sz, pos;
> void *addr;
> char c;
> int fd, ret;
>
> if (argc != 2) {
> printf("Usage: %s fname\n", argv[0]);
> return 1;
> }
>
> fd = open(argv[1], O_RDONLY);
> if (fd < 0) {
> perror(argv[1]);
> return 1;
> }
>
> ret = fstat(fd, &sb);
> if (ret) {
> perror("fstat");
> return 1;
> }
>
> /* Validate the file contents with regular reads */
> for (pos = 0; pos < sb.st_size; pos += sb.st_blksize) {
> ret = pread(fd, &c, 1, pos);
> if (ret < 0) {
> if (errno != EIO) {
> perror("pread");
> return 1;
> }
>
> printf("%s: at offset %llu: %s\n", argv[1],
> (unsigned long long)pos,
> strerror(errno));
> break;
> }
> }
>
> ret = pread(fd, &c, 1, sb.st_size);
> if (ret < 0) {
> if (errno != EIO) {
> perror("pread");
> return 1;
> }
>
> printf("%s: at offset %llu: %s\n", argv[1],
> (unsigned long long)sb.st_size,
> strerror(errno));
> }
>
> /* Validate the file contents with MADV_POPULATE_READ */
> read_sz = ((sb.st_size + (pagesize - 1)) / pagesize) * pagesize;
> printf("%s: read bytes %llu\n", argv[1], (unsigned long long)read_sz);
>
> for (pos = 0; pos < read_sz; pos += BUFSIZE) {
> unsigned int mappos;
> size_t maplen = min(read_sz - pos, BUFSIZE);
>
> addr = mmap(NULL, maplen, PROT_READ, MAP_SHARED, fd, pos);
> if (addr == MAP_FAILED) {
> perror("mmap");
> return 1;
> }
>
> ret = madvise(addr, maplen, MADV_POPULATE_READ);
> if (ret) {
> perror("madvise");
> return 1;
> }
>
> ret = munmap(addr, maplen);
> if (ret) {
> perror("munmap");
> return 1;
> }
> }
>
> ret = close(fd);
> if (ret) {
> perror("close");
> return 1;
> }
>
> return 0;
> }
>
> and this shell script mpr.sh:
>
> #!/bin/bash -x
>
> # Try to trigger infinite loop with regular IO errors and MADV_POPULATE_READ
>
> scriptdir="$(dirname "$0")"
>
> commands=(dmsetup mkfs.xfs xfs_io timeout strace "$scriptdir/mpr")
> for cmd in "${commands[@]}"; do
> if ! command -v "$cmd" &>/dev/null; then
> echo "$cmd: Command required for this program."
> exit 1
> fi
> done
>
> dev="${1:-/dev/sda}"
> mnt="${2:-/mnt}"
> dmtarget="dumbtarget"
>
> # Clean up any old mounts
> umount "$dev" "$mnt"
> dmsetup remove "$dmtarget"
> rmmod xfs
>
> # Create dm linear mapping to block device and format filesystem
> sectors="$(blockdev --getsz "$dev")"
> tgt="/dev/mapper/$dmtarget"
> echo "0 $sectors linear $dev 0" | dmsetup create "$dmtarget"
> mkfs.xfs -f "$tgt"
>
> # Create a file that we'll read, then cycle mount to zap pagecache
> mount "$tgt" "$mnt"
> xfs_io -f -c "pwrite -S 0x58 0 1m" "$mnt/a"
> umount "$mnt"
> mount "$tgt" "$mnt"
>
> # Load file metadata
> stat "$mnt/a"
>
> # Induce EIO errors on read
> dmsetup suspend --noflush --nolockfs "$dmtarget"
> echo "0 $sectors error" | dmsetup load "$dmtarget"
> dmsetup resume "$dmtarget"
>
> # Try to provoke the kernel; kill the process after 10s so we can clean up
> timeout -s KILL 10s strace -s99 -e madvise "$scriptdir/mpr" "$mnt/a"
>
> # Stop EIO errors so we can unmount
> dmsetup suspend --noflush --nolockfs "$dmtarget"
> echo "0 $sectors linear $dev 0" | dmsetup load "$dmtarget"
> dmsetup resume "$dmtarget"
>
> # Unmount and clean up after ourselves
> umount "$mnt"
> dmsetup remove "$dmtarget"
> <EOF>
>
> make the C program, then run ./mpr.sh <device> <mountpoint>. It should
> stall in the madvise call until timeout sends sigkill to the program;
> you can crank the 10s timeout up if you want.
>
> <insert usual disclaimer that I run all these things in scratch VMs>

Yes, seems to work, nice!

[ 452.455636] buffer_io_error: 6 callbacks suppressed
[ 452.455638] Buffer I/O error on dev dm-0, logical block 16, async page read
[ 452.456169] Buffer I/O error on dev dm-0, logical block 17, async page read
[ 452.456456] Buffer I/O error on dev dm-0, logical block 18, async page read
[ 452.456754] Buffer I/O error on dev dm-0, logical block 19, async page read
[ 452.457061] Buffer I/O error on dev dm-0, logical block 20, async page read
[ 452.457350] Buffer I/O error on dev dm-0, logical block 21, async page read
[ 452.457639] Buffer I/O error on dev dm-0, logical block 22, async page read
[ 452.457942] Buffer I/O error on dev dm-0, logical block 23, async page read
[ 452.458242] Buffer I/O error on dev dm-0, logical block 16, async page read
[ 452.458552] Buffer I/O error on dev dm-0, logical block 17, async page read
+ timeout -s KILL 10s strace -s99 -e madvise ./mpr /mnt/tmp//a
/mnt/tmp//a: at offset 0: Input/output error
/mnt/tmp//a: read bytes 1048576
madvise(0x7f9393624000, 1048576, MADV_POPULATE_READ./mpr.sh: line 45: 2070 Killed tim"

And once I switch over to reading instead of MADV_POPULATE_READ:

[ 753.940230] buffer_io_error: 6 callbacks suppressed
[ 753.940233] Buffer I/O error on dev dm-0, logical block 8192, async page read
[ 753.941402] Buffer I/O error on dev dm-0, logical block 8193, async page read
[ 753.942084] Buffer I/O error on dev dm-0, logical block 8194, async page read
[ 753.942738] Buffer I/O error on dev dm-0, logical block 8195, async page read
[ 753.943412] Buffer I/O error on dev dm-0, logical block 8196, async page read
[ 753.944088] Buffer I/O error on dev dm-0, logical block 8197, async page read
[ 753.944741] Buffer I/O error on dev dm-0, logical block 8198, async page read
[ 753.945415] Buffer I/O error on dev dm-0, logical block 8199, async page read
[ 753.946105] Buffer I/O error on dev dm-0, logical block 8192, async page read
[ 753.946661] Buffer I/O error on dev dm-0, logical block 8193, async page read
+ timeout -s KILL 10s strace -s99 -e madvise ./mpr /mnt/tmp//a
/mnt/tmp//a: at offset 0: Input/output error
/mnt/tmp//a: read bytes 1048576
--- SIGBUS {si_signo=SIGBUS, si_code=BUS_ADRERR, si_addr=0x7f34f82d8000} ---
+++ killed by SIGBUS (core dumped) +++
timeout: the monitored command dumped core
./mpr.sh: line 45: 2388 Bus error timeout -s KILL 10s strace -s99 -e madvise "$scriptdir"

Let me dig how the fault handler is able to conclude SIGBUS here!

--
Cheers,

David / dhildenb