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 ]---
Signed-off-by: Axel Rasmussen axelrasmussen@google.com Cc: stable@vger.kernel.org --- mm/secretmem.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)
diff --git a/mm/secretmem.c b/mm/secretmem.c index 098638d3b8a4..a83e98aa3a7b 100644 --- a/mm/secretmem.c +++ b/mm/secretmem.c @@ -158,6 +158,22 @@ const struct address_space_operations secretmem_aops = { .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_create(unsigned long flags) 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 */
On Thu, Mar 24, 2022 at 02:09:09PM -0700, Axel Rasmussen wrote:
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).
Isn't ENOTTY the normal return value for this? Or even ENOSYS?
On Thu, Mar 24, 2022 at 2:33 PM Matthew Wilcox willy@infradead.org wrote:
On Thu, Mar 24, 2022 at 02:09:09PM -0700, Axel Rasmussen wrote:
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).
Isn't ENOTTY the normal return value for this? Or even ENOSYS?
I'm unsure.
Since errno(3) says ENOTTY means "Inappropriate I/O control operation" that makes me think it's meant to be used only for ioctls?
I tried ENOSYS, but checkpatch warns me it's meant to be used for "invalid syscall nr" and nothing else.
ENOTSUP / ENOTSUPP / EOPNOTSUPP all have their own share of weirdnesses too, though. There's the whole ENOTSUP / ENOTSUPP mess, and then also the fact that glibc says ENOTSUP == EOPNOTSUPP, whereas POSIX says EOPNOTSUPP should be distinct and used specifically for sockets...
Any strong opinions on which error code is used? I think overall I would still pick EOPNOTSUPP, but happy to change it if anyone feels strongly.
- I think ENOSYS is specific to syscall nr not defined - I think ENOTTY is specific to ioctls - The kernel (sort of mistakenly) defines ENOTSUPP instead of ENOTSUP, but it's marked deprecated and it's recommended to use EOPNOTSUPP instead (despite POSIX saying these should be distinct and for different uses).
On Thu, Mar 24, 2022 at 2:44 PM Axel Rasmussen axelrasmussen@google.com wrote:
On Thu, Mar 24, 2022 at 2:33 PM Matthew Wilcox willy@infradead.org wrote:
On Thu, Mar 24, 2022 at 02:09:09PM -0700, Axel Rasmussen wrote:
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).
Isn't ENOTTY the normal return value for this? Or even ENOSYS?
I'm unsure.
Since errno(3) says ENOTTY means "Inappropriate I/O control operation" that makes me think it's meant to be used only for ioctls?
I tried ENOSYS, but checkpatch warns me it's meant to be used for "invalid syscall nr" and nothing else.
ENOTSUP / ENOTSUPP / EOPNOTSUPP all have their own share of weirdnesses too, though. There's the whole ENOTSUP / ENOTSUPP mess, and then also the fact that glibc says ENOTSUP == EOPNOTSUPP, whereas POSIX says EOPNOTSUPP should be distinct and used specifically for sockets...
On Thu, 31 Mar 2022 10:42:12 -0700 Axel Rasmussen axelrasmussen@google.com wrote:
Any strong opinions on which error code is used? I think overall I would still pick EOPNOTSUPP, but happy to change it if anyone feels strongly.
- I think ENOSYS is specific to syscall nr not defined
- I think ENOTTY is specific to ioctls
- The kernel (sort of mistakenly) defines ENOTSUPP instead of ENOTSUP,
but it's marked deprecated and it's recommended to use EOPNOTSUPP instead (despite POSIX saying these should be distinct and for different uses).
`man ftruncate' sayeth
EINVAL The argument length is negative or larger than the maximum file size.
which reasonably accurately describes what we're doing here?
+ if ((ia_valid & ATTR_SIZE) && inode->i_size) + return -EOPNOTSUPP;
It's a reasonable option.
Worth noting though that today when it says "larger than the maximum file size", it refers to the global MAX_NON_LFS (2^31), not necessarily growing in general. So, we're extending the meaning a bit.
But, I'm happy to send a v2 with EINVAL instead.
On Thu, Apr 7, 2022 at 1:50 PM Andrew Morton akpm@linux-foundation.org wrote:
On Thu, 31 Mar 2022 10:42:12 -0700 Axel Rasmussen axelrasmussen@google.com wrote:
Any strong opinions on which error code is used? I think overall I would still pick EOPNOTSUPP, but happy to change it if anyone feels strongly.
- I think ENOSYS is specific to syscall nr not defined
- I think ENOTTY is specific to ioctls
- The kernel (sort of mistakenly) defines ENOTSUPP instead of ENOTSUP,
but it's marked deprecated and it's recommended to use EOPNOTSUPP instead (despite POSIX saying these should be distinct and for different uses).
`man ftruncate' sayeth
EINVAL The argument length is negative or larger than the maximum file size.which reasonably accurately describes what we're doing here?
if ((ia_valid & ATTR_SIZE) && inode->i_size)return -EOPNOTSUPP;
linux-stable-mirror@lists.linaro.org
-
Andrew Morton -
Axel Rasmussen -
Matthew Wilcox