CVE-2024-35877 Information

May 20, 2024 cve

Description

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] [ 301.566325] ? untrack_pfn+0xf4/0x100 [ 301.566618] ? __warn+0x81/0x130 [ 301.566876] ? untrack_pfn+0xf4/0x100 [ 3

truncated—

Reference

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