The patch titled Subject: mm/secretmem: fix panic when growing a memfd_secret has been added to the -mm tree. Its filename is mm-secretmem-fix-panic-when-growing-a-memfd_secret.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-secretmem-fix-panic-when-growin... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-secretmem-fix-panic-when-growin...

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------------------------------------------------------ From: Axel Rasmussen axelrasmussen@google.com Subject: mm/secretmem: fix panic when growing a memfd_secret

When one tries to grow an existing memfd_secret with ftruncate, one gets a panic [1]. For example, doing the following reliably induces the panic:

fd = memfd_secret();

ftruncate(fd, 10); ptr = mmap(NULL, 10, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); strcpy(ptr, "123456789");

munmap(ptr, 10); ftruncate(fd, 20);

The basic reason for this is, when we grow with ftruncate, we call down into simple_setattr, and then truncate_inode_pages_range, and eventually we try to zero part of the memory. The normal truncation code does this via the direct map (i.e., it calls page_address() and hands that to memset()).

For memfd_secret though, we specifically don't map our pages via the direct map (i.e. we call set_direct_map_invalid_noflush() on every fault). So the address returned by page_address() isn't useful, and when we try to memset() with it we panic.

This patch avoids the panic by implementing a custom setattr for memfd_secret, which detects resizes specifically (setting the size for the first time works just fine, since there are no existing pages to try to zero), and rejects them as not supported (ENOTSUP).

One could argue growing should be supported, but I think that will require a significantly more lengthy change. So, I propose a minimal fix for the benefit of stable kernels, and then perhaps to extend memfd_secret to support growing in a separate patch.

[1]:

[ 774.320433] BUG: unable to handle page fault for address: ffffa0a889277028 [ 774.322297] #PF: supervisor write access in kernel mode [ 774.323306] #PF: error_code(0x0002) - not-present page [ 774.324296] PGD afa01067 P4D afa01067 PUD 83f909067 PMD 83f8bf067 PTE 800ffffef6d88060 [ 774.325841] Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI [ 774.326934] CPU: 0 PID: 281 Comm: repro Not tainted 5.17.0-dbg-DEV #1 [ 774.328074] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 [ 774.329732] RIP: 0010:memset_erms+0x9/0x10 [ 774.330474] Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01 [ 774.333543] RSP: 0018:ffffb932c09afbf0 EFLAGS: 00010246 [ 774.334404] RAX: 0000000000000000 RBX: ffffda63c4249dc0 RCX: 0000000000000fd8 [ 774.335545] RDX: 0000000000000fd8 RSI: 0000000000000000 RDI: ffffa0a889277028 [ 774.336685] RBP: ffffb932c09afc00 R08: 0000000000001000 R09: ffffa0a889277028 [ 774.337929] R10: 0000000000020023 R11: 0000000000000000 R12: ffffda63c4249dc0 [ 774.339236] R13: ffffa0a890d70d98 R14: 0000000000000028 R15: 0000000000000fd8 [ 774.340356] FS: 00007f7294899580(0000) GS:ffffa0af9bc00000(0000) knlGS:0000000000000000 [ 774.341635] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 774.342535] CR2: ffffa0a889277028 CR3: 0000000107ef6006 CR4: 0000000000370ef0 [ 774.343651] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 774.344780] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 774.345938] Call Trace: [ 774.346334] <TASK> [ 774.346671] ? zero_user_segments+0x82/0x190 [ 774.347346] truncate_inode_partial_folio+0xd4/0x2a0 [ 774.348128] truncate_inode_pages_range+0x380/0x830 [ 774.348904] truncate_setsize+0x63/0x80 [ 774.349530] simple_setattr+0x37/0x60 [ 774.350102] notify_change+0x3d8/0x4d0 [ 774.350681] do_sys_ftruncate+0x162/0x1d0 [ 774.351302] __x64_sys_ftruncate+0x1c/0x20 [ 774.351936] do_syscall_64+0x44/0xa0 [ 774.352486] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 774.353284] RIP: 0033:0x7f72947c392b [ 774.354001] Code: 77 05 c3 0f 1f 40 00 48 8b 15 41 85 0c 00 f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa b8 4d 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 11 85 0c 00 f7 d8 [ 774.357938] RSP: 002b:00007ffcad62a1a8 EFLAGS: 00000202 ORIG_RAX: 000000000000004d [ 774.359116] RAX: ffffffffffffffda RBX: 000055f47662b440 RCX: 00007f72947c392b [ 774.360186] RDX: 0000000000000028 RSI: 0000000000000028 RDI: 0000000000000003 [ 774.361246] RBP: 00007ffcad62a1c0 R08: 0000000000000003 R09: 0000000000000000 [ 774.362324] R10: 00007f72946dc230 R11: 0000000000000202 R12: 000055f47662b0e0 [ 774.363393] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 [ 774.364470] </TASK> [ 774.364807] Modules linked in: xhci_pci xhci_hcd virtio_net net_failover failover virtio_blk virtio_balloon uhci_hcd ohci_pci ohci_hcd evdev ehci_pci ehci_hcd 9pnet_virtio 9p netfs 9pnet [ 774.367325] CR2: ffffa0a889277028 [ 774.367838] ---[ end trace 0000000000000000 ]--- [ 774.368543] RIP: 0010:memset_erms+0x9/0x10 [ 774.369187] Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01 [ 774.372282] RSP: 0018:ffffb932c09afbf0 EFLAGS: 00010246 [ 774.373372] RAX: 0000000000000000 RBX: ffffda63c4249dc0 RCX: 0000000000000fd8 [ 774.374814] RDX: 0000000000000fd8 RSI: 0000000000000000 RDI: ffffa0a889277028 [ 774.376248] RBP: ffffb932c09afc00 R08: 0000000000001000 R09: ffffa0a889277028 [ 774.377687] R10: 0000000000020023 R11: 0000000000000000 R12: ffffda63c4249dc0 [ 774.379135] R13: ffffa0a890d70d98 R14: 0000000000000028 R15: 0000000000000fd8 [ 774.380550] FS: 00007f7294899580(0000) GS:ffffa0af9bc00000(0000) knlGS:0000000000000000 [ 774.382177] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 774.383329] CR2: ffffa0a889277028 CR3: 0000000107ef6006 CR4: 0000000000370ef0 [ 774.384763] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 774.386229] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 774.387664] Kernel panic - not syncing: Fatal exception [ 774.388863] Kernel Offset: 0x8000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff) [ 774.391014] ---[ end Kernel panic - not syncing: Fatal exception ]---

Link: https://lkml.kernel.org/r/20220324210909.1843814-1-axelrasmussen@google.com Signed-off-by: Axel Rasmussen axelrasmussen@google.com Cc: Mike Rapoport rppt@kernel.org Cc: Matthew Wilcox willy@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/secretmem.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)

--- a/mm/secretmem.c~mm-secretmem-fix-panic-when-growing-a-memfd_secret +++ a/mm/secretmem.c @@ -158,6 +158,22 @@ const struct address_space_operations se .isolate_page = secretmem_isolate_page, };

+static int secretmem_setattr(struct user_namespace *mnt_userns, + struct dentry *dentry, struct iattr *iattr) +{ + struct inode *inode = d_inode(dentry); + unsigned int ia_valid = iattr->ia_valid; + + if ((ia_valid & ATTR_SIZE) && inode->i_size) + return -EOPNOTSUPP; + + return simple_setattr(mnt_userns, dentry, iattr); +} + +const struct inode_operations secretmem_iops = { + .setattr = secretmem_setattr, +}; + static struct vfsmount *secretmem_mnt;

static struct file *secretmem_file_create(unsigned long flags) @@ -177,6 +193,7 @@ static struct file *secretmem_file_creat mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER); mapping_set_unevictable(inode->i_mapping);

+ inode->i_op = &secretmem_iops; inode->i_mapping->a_ops = &secretmem_aops;

/* pretend we are a normal file with zero size */ _

Patches currently in -mm which might be from axelrasmussen@google.com are

mm-secretmem-fix-panic-when-growing-a-memfd_secret.patch