CVE-2024-35877: In the Linux kernel, the following vulnerability has been resolved:  x86/mm/pat: fix VM_PAT handling in COW mappings  PAT handling won't do the right thing in COW mappings: the first PTE (or, in fact, all PTEs) can be replaced during write faults to point at anon folios.  Reliably recovering the correct PFN and cachemode using follow_phys() from PTEs will not work in COW mappings.  Using follow_phys(), we might just get the address+protection of the anon folio (which is very wrong), or fail on swap/nonswap entries, failing follow_phys() and triggering a WARN_ON_ONCE() in untrack_pfn() and track_pfn_copy(), not properly calling free_pfn_range().  In free_pfn_range(), we either wouldn't call memtype_free() or would call it with the wrong range, possibly leaking memory.  To fix that, let's update follow_phys() to refuse returning anon folios, and fallback to using the stored PFN inside vma->vm_pgoff for COW mappings if we run into that.  We will now properly handle untrack_pfn() with COW mappings, where we don't need the cachemode.  We'll have to fail fork()->track_pfn_copy() if the first page was replaced by an anon folio, though: we'd have to store the cachemode in the VMA to make this work, likely growing the VMA size.  For now, lets keep it simple and let track_pfn_copy() just fail in that case: it would have failed in the past with swap/nonswap entries already, and it would have done the wrong thing with anon folios.  Simple reproducer to trigger the WARN_ON_ONCE() in untrack_pfn():  <--- C reproducer --->  #include <stdio.h>  #include <sys/mman.h>  #include <unistd.h>  #include <liburing.h>   int main(void)  {          struct io_uring_params p = {};          int ring_fd;          size_t size;          char *map;           ring_fd = io_uring_setup(1, &p);          if (ring_fd < 0) {                  perror("io_uring_setup");                  return 1;          }          size = p.sq_off.array + p.sq_entries * sizeof(unsigned);           /* Map the submission queue ring MAP_PRIVATE */          map = mmap(0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE,                     ring_fd, IORING_OFF_SQ_RING);          if (map == MAP_FAILED) {                  perror("mmap");                  return 1;          }           /* We have at least one page. Let's COW it. */          *map = 0;          pause();          return 0;  } <--- C reproducer --->  On a system with 16 GiB RAM and swap configured:  # ./iouring &  # memhog 16G  # killall iouring [  301.552930] ------------[ cut here ]------------ [  301.553285] WARNING: CPU: 7 PID: 1402 at arch/x86/mm/pat/memtype.c:1060 untrack_pfn+0xf4/0x100 [  301.553989] Modules linked in: binfmt_misc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_g [  301.558232] CPU: 7 PID: 1402 Comm: iouring Not tainted 6.7.5-100.fc38.x86_64 #1 [  301.558772] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebu4 [  301.559569] RIP: 0010:untrack_pfn+0xf4/0x100 [  301.559893] Code: 75 c4 eb cf 48 8b 43 10 8b a8 e8 00 00 00 3b 6b 28 74 b8 48 8b 7b 30 e8 ea 1a f7 000 [  301.561189] RSP: 0018:ffffba2c0377fab8 EFLAGS: 00010282 [  301.561590] RAX: 00000000ffffffea RBX: ffff9208c8ce9cc0 RCX: 000000010455e047 [  301.562105] RDX: 07fffffff0eb1e0a RSI: 0000000000000000 RDI: ffff9208c391d200 [  301.562628] RBP: 0000000000000000 R08: ffffba2c0377fab8 R09: 0000000000000000 [  301.563145] R10: ffff9208d2292d50 R11: 0000000000000002 R12: 00007fea890e0000 [  301.563669] R13: 0000000000000000 R14: ffffba2c0377fc08 R15: 0000000000000000 [  301.564186] FS:  0000000000000000(0000) GS:ffff920c2fbc0000(0000) knlGS:0000000000000000 [  301.564773] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [  301.565197] CR2: 00007fea88ee8a20 CR3: 00000001033a8000 CR4: 0000000000750ef0 [  301.565725] PKRU: 55555554 [  301.565944] Call Trace: [  301.566148]  <TASK> [  301.566325]  ? untrack_pfn+0xf4/0x100 [  301.566618]  ? __warn+0x81/0x130 [  301.566876]  ? untrack_pfn+0xf4/0x100 [  3 ---truncated---