CVE-2024-38599: jffs2: prevent xattr node from overflowing the eraseblock

In the Linux kernel, the following vulnerability has been resolved:

jffs2: prevent xattr node from overflowing the eraseblock

Add a check to make sure that the requested xattr node size is no larger than the eraseblock minus the cleanmarker.

Unlike the usual inode nodes, the xattr nodes aren't split into parts and spread across multiple eraseblocks, which means that a xattr node must not occupy more than one eraseblock. If the requested xattr value is too large, the xattr node can spill onto the next eraseblock, overwriting the nodes and causing errors such as:

jffs2: argh. node added in wrong place at 0x0000b050(2) jffs2: nextblock 0x0000a000, expected at 0000b00c jffs2: error: (823) doverifyxattrdatum: node CRC failed at 0x01e050, read=0xfc892c93, calc=0x000000 jffs2: notice: (823) jffs2getinodenodes: Node header CRC failed at 0x01e00c. {848f,2fc4,0fef511f,59a3d171} jffs2: Node at 0x0000000c with length 0x00001044 would run over the end of the erase block jffs2: Perhaps the file system was created with the wrong erase size? jffs2: jffs2scaneraseblock(): Magic bitmask 0x1985 not found at 0x00000010: 0x1044 instead

This breaks the filesystem and can lead to KASAN crashes such as:

BUG: KASAN: slab-out-of-bounds in jffs2sumaddkvec+0x125e/0x15d0 Read of size 4 at addr ffff88802c31e914 by task repro/830 CPU: 0 PID: 830 Comm: repro Not tainted 6.9.0-rc3+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014 Call Trace: <TASK> dumpstacklvl+0xc6/0x120 printreport+0xc4/0x620 ? virtaddrvalid+0x308/0x5b0 kasanreport+0xc1/0xf0 ? jffs2sumaddkvec+0x125e/0x15d0 ? jffs2sumaddkvec+0x125e/0x15d0 jffs2sumaddkvec+0x125e/0x15d0 jffs2flashdirectwritev+0xa8/0xd0 jffs2flashwritev+0x9c9/0xef0 ? x64syssetxattr+0xc4/0x160 ? dosyscall64+0x69/0x140 ? entrySYSCALL64afterhwframe+0x76/0x7e [...]

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.