[release-branch.go1.19] mime/multipart: limit memory/inode consumptio…

…n of ReadForm

Reader.ReadForm is documented as storing "up to maxMemory bytes + 10MB"
in memory. Parsed forms can consume substantially more memory than
this limit, since ReadForm does not account for map entry overhead
and MIME headers.

In addition, while the amount of disk memory consumed by ReadForm can
be constrained by limiting the size of the parsed input, ReadForm will
create one temporary file per form part stored on disk, potentially
consuming a large number of inodes.

Update ReadForm's memory accounting to include part names,
MIME headers, and map entry overhead.

Update ReadForm to store all on-disk file parts in a single
temporary file.

Files returned by FileHeader.Open are documented as having a concrete
type of *os.File when a file is stored on disk. The change to use a
single temporary file for all parts means that this is no longer the
case when a form contains more than a single file part stored on disk.

The previous behavior of storing each file part in a separate disk
file may be reenabled with GODEBUG=multipartfiles=distinct.

Update Reader.NextPart and Reader.NextRawPart to set a 10MiB cap
on the size of MIME headers.

Thanks to Jakob Ackermann (@das7pad) for reporting this issue.

Updates #58006
Fixes #58362
Fixes CVE-2022-41725

Change-Id: Ibd780a6c4c83ac8bcfd3cbe344f042e9940f2eab
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1714276
Reviewed-by: Julie Qiu <julieqiu@google.com>
TryBot-Result: Security TryBots <security-trybots@go-security-trybots.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
(cherry picked from commit ed4664330edcd91b24914c9371c377c132dbce8c)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1728949
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/468116
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

request_test.go

@@ -1104,7 +1104,7 @@ func testMissingFile(t *testing.T, req *Request) {
 		t.Errorf("FormFile file = %v, want nil", f)
 	}
 	if fh != nil {
-		t.Errorf("FormFile file header = %q, want nil", fh)
+		t.Errorf("FormFile file header = %v, want nil", fh)
 	}
 	if err != ErrMissingFile {
 		t.Errorf("FormFile err = %q, want ErrMissingFile", err)