pstore/ram: Correctly calculate usable PRZ bytes

The actual number of bytes stored in a PRZ is smaller than the
bytes requested by platform data, since there is a header on each
PRZ. Additionally, if ECC is enabled, there are trailing bytes used
as well. Normally this mismatch doesn't matter since PRZs are circular
buffers and the leading "overflow" bytes are just thrown away. However, in
the case of a compressed record, this rather badly corrupts the results.

This corruption was visible with "ramoops.mem_size=204800 ramoops.ecc=1".
Any stored crashes would not be uncompressable (producing a pstorefs
"dmesg-*.enc.z" file), and triggering errors at boot:

  [    2.790759] pstore: crypto_comp_decompress failed, ret = -22!

Backporting this depends on commit 70ad35db ("pstore: Convert console
write to use ->write_buf")
Reported-by: Joel Fernandes <joel@joelfernandes.org>
Fixes: b0aad7a9 ("pstore: Add compression support to pstore")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

fs/pstore/ram.c

@@ -816,17 +816,14 @@ static int ramoops_probe(struct platform_device *pdev)
 
 	cxt->pstore.data = cxt;
 	/*
-	 * Console can handle any buffer size, so prefer LOG_LINE_MAX. If we
-	 * have to handle dumps, we must have at least record_size buffer. And
-	 * for ftrace, bufsize is irrelevant (if bufsize is 0, buf will be
-	 * ZERO_SIZE_PTR).
+	 * Since bufsize is only used for dmesg crash dumps, it
+	 * must match the size of the dprz record (after PRZ header
+	 * and ECC bytes have been accounted for).
 	 */
-	if (cxt->console_size)
-		cxt->pstore.bufsize = 1024; /* LOG_LINE_MAX */
-	cxt->pstore.bufsize = max(cxt->record_size, cxt->pstore.bufsize);
-	cxt->pstore.buf = kmalloc(cxt->pstore.bufsize, GFP_KERNEL);
+	cxt->pstore.bufsize = cxt->dprzs[0]->buffer_size;
+	cxt->pstore.buf = kzalloc(cxt->pstore.bufsize, GFP_KERNEL);
 	if (!cxt->pstore.buf) {
-		pr_err("cannot allocate pstore buffer\n");
+		pr_err("cannot allocate pstore crash dump buffer\n");
 		err = -ENOMEM;
 		goto fail_clear;
 	}

include/linux/pstore.h

@@ -90,7 +90,10 @@ struct pstore_record {
  *
  * @buf_lock:	spinlock to serialize access to @buf
  * @buf:	preallocated crash dump buffer
- * @bufsize:	size of @buf available for crash dump writes
+ * @bufsize:	size of @buf available for crash dump bytes (must match
+ *		smallest number of bytes available for writing to a
+ *		backend entry, since compressed bytes don't take kindly
+ *		to being truncated)
  *
  * @read_mutex:	serializes @open, @read, @close, and @erase callbacks
  * @flags:	bitfield of frontends the backend can accept writes for