journal: limit decompress_blob() output to DATA_SIZE_MAX

We already have checks in place during compression that limit the data
we compress, so they shouldn't decompress to anything larger than
DATA_SIZE_MAX unless they've been tampered with. Let's make this
explicit and limit all our decompress_blob() calls in journal-handling
code to that limit.

One possible scenario this should prevent is when one tries to open and
verify a journal file that contains a compression bomb in its payload:

$ ls -lh test.journal
-rw-rw-r--+ 1 fsumsal fsumsal 1.2M Apr 12 15:07 test.journal

$ systemd-run --user --wait --pipe -- build-local/journalctl --verify --file=$PWD/test.journal
Running as unit: run-p682422-i4875779.service
000110: Invalid hash (00000000 vs. 11e4948d73bdafdd)
000110: Invalid object contents: Bad message
File corruption detected at /home/fsumsal/repos/@systemd/systemd/test.journal:272 (of 1249896 bytes, 0%).
FAIL: /home/fsumsal/repos/@systemd/systemd/test.journal (Bad message)
          Finished with result: exit-code
Main processes terminated with: code=exited, status=1/FAILURE
               Service runtime: 48.051s
             CPU time consumed: 47.941s
                   Memory peak: 8G (swap: 0B)

Same could be, in theory, possible with just `journalctl --file=`, but
the reproducer would be a bit more complicated (haven't tried it, yet).

Lastly, the change in journal-remote is mostly hardening, as the maximum
input size to decompress_blob() there is mandated by MHD's connection
memory limit (set to JOURNAL_SERVER_MEMORY_MAX which is 128 KiB at the
time of writing), so the possible output size there is already quite
limited (e.g. ~800 - 900 MiB for xz-compressed data).

src/journal-remote/journal-remote-main.c

@@ -286,7 +286,7 @@ static int process_http_upload(
                         _cleanup_free_ char *buf = NULL;
                         size_t buf_size;
 
-                        r = decompress_blob(source->compression, upload_data, *upload_data_size, (void **) &buf, &buf_size, 0);
+                        r = decompress_blob(source->compression, upload_data, *upload_data_size, (void **) &buf, &buf_size, DATA_SIZE_MAX);
                         if (r < 0)
                                 return mhd_respondf(connection, r, MHD_HTTP_BAD_REQUEST, "Decompression of received blob failed.");
 

src/libsystemd/sd-journal/journal-file.c

@@ -1971,7 +1971,7 @@ static int maybe_decompress_payload(
                                 return 1;
                 }
 
-                r = decompress_blob(compression, payload, size, &f->compress_buffer, &rsize, 0);
+                r = decompress_blob(compression, payload, size, &f->compress_buffer, &rsize, DATA_SIZE_MAX);
                 if (r < 0)
                         return r;
 

src/libsystemd/sd-journal/journal-verify.c

@@ -126,7 +126,7 @@ static int hash_payload(JournalFile *f, Object *o, uint64_t offset, const uint8_
                 _cleanup_free_ void *b = NULL;
                 size_t b_size;
 
-                r = decompress_blob(c, src, size, &b, &b_size, 0);
+                r = decompress_blob(c, src, size, &b, &b_size, DATA_SIZE_MAX);
                 if (r < 0) {
                         error_errno(offset, r, "%s decompression failed: %m",
                                     compression_to_string(c));