CVE-2024-56611

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

mm/mempolicy: fix migratetonode() assuming there is at least one VMA in a MM

We currently assume that there is at least one VMA in a MM, which isn't true.

So we might end up having findvma() return NULL, to then de-reference NULL. So properly handle findvma() returning NULL.

This fixes the report:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] CPU: 1 UID: 0 PID: 6021 Comm: syz-executor284 Not tainted 6.12.0-rc7-syzkaller-00187-gf868cd251776 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024 RIP: 0010:migratetonode mm/mempolicy.c:1090 [inline] RIP: 0010:domigratepages+0x403/0x6f0 mm/mempolicy.c:1194 Code: ... RSP: 0018:ffffc9000375fd08 EFLAGS: 00010246 RAX: 0000000000000000 RBX: ffffc9000375fd78 RCX: 0000000000000000 RDX: ffff88807e171300 RSI: dffffc0000000000 RDI: ffff88803390c044 RBP: ffff88807e171428 R08: 0000000000000014 R09: fffffbfff2039ef1 R10: ffffffff901cf78f R11: 0000000000000000 R12: 0000000000000003 R13: ffffc9000375fe90 R14: ffffc9000375fe98 R15: ffffc9000375fdf8 FS: 00005555919e1380(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00005555919e1ca8 CR3: 000000007f12a000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> kernelmigratepages+0x5b2/0x750 mm/mempolicy.c:1709 __dosysmigrate_pages mm/mempolicy.c:1727 [inline] __sesysmigrate_pages mm/mempolicy.c:1723 [inline] _x64sysmigratepages+0x96/0x100 mm/mempolicy.c:1723 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xcd/0x250 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f

[akpm@linux-foundation.org: add unlikely()]