Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK
From: Mike Kravetz
Date: Fri Aug 04 2017 - 19:10:16 EST
On 08/04/2017 12:07 PM, riel@xxxxxxxxxx wrote:
> From: Rik van Riel <riel@xxxxxxxxxx>
>
> Introduce MADV_WIPEONFORK semantics, which result in a VMA being
> empty in the child process after fork. This differs from MADV_DONTFORK
> in one important way.
>
> If a child process accesses memory that was MADV_WIPEONFORK, it
> will get zeroes. The address ranges are still valid, they are just empty.
>
> If a child process accesses memory that was MADV_DONTFORK, it will
> get a segmentation fault, since those address ranges are no longer
> valid in the child after fork.
>
> Since MADV_DONTFORK also seems to be used to allow very large
> programs to fork in systems with strict memory overcommit restrictions,
> changing the semantics of MADV_DONTFORK might break existing programs.
>
> The use case is libraries that store or cache information, and
> want to know that they need to regenerate it in the child process
> after fork.
>
> Examples of this would be:
> - systemd/pulseaudio API checks (fail after fork)
> (replacing a getpid check, which is too slow without a PID cache)
> - PKCS#11 API reinitialization check (mandated by specification)
> - glibc's upcoming PRNG (reseed after fork)
> - OpenSSL PRNG (reseed after fork)
>
> The security benefits of a forking server having a re-inialized
> PRNG in every child process are pretty obvious. However, due to
> libraries having all kinds of internal state, and programs getting
> compiled with many different versions of each library, it is
> unreasonable to expect calling programs to re-initialize everything
> manually after fork.
>
> A further complication is the proliferation of clone flags,
> programs bypassing glibc's functions to call clone directly,
> and programs calling unshare, causing the glibc pthread_atfork
> hook to not get called.
>
> It would be better to have the kernel take care of this automatically.
>
> This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO:
>
> https://man.openbsd.org/minherit.2
>
> Reported-by: Florian Weimer <fweimer@xxxxxxxxxx>
> Reported-by: Colm MacCÃrtaigh <colm@xxxxxxxxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> arch/alpha/include/uapi/asm/mman.h | 3 +++
> arch/mips/include/uapi/asm/mman.h | 3 +++
> arch/parisc/include/uapi/asm/mman.h | 3 +++
> arch/xtensa/include/uapi/asm/mman.h | 3 +++
> fs/proc/task_mmu.c | 1 +
> include/linux/mm.h | 2 +-
> include/uapi/asm-generic/mman-common.h | 3 +++
> kernel/fork.c | 8 ++++++--
> mm/madvise.c | 8 ++++++++
> mm/memory.c | 10 ++++++++++
> 10 files changed, 41 insertions(+), 3 deletions(-)
>
This didn't seem 'quite right' to me for shared mappings and/or file
backed mappings. I wasn't exactly sure what it 'should' do in such
cases. So, I tried it with a mapping created as follows:
addr = mmap(ADDR, page_size,
PROT_READ | PROT_WRITE,
MAP_ANONYMOUS|MAP_SHARED, -1, 0);
When setting MADV_WIPEONFORK on the vma/mapping, I got the following
at task exit time:
[ 694.558290] ------------[ cut here ]------------
[ 694.558978] kernel BUG at mm/filemap.c:212!
[ 694.559476] invalid opcode: 0000 [#1] SMP
[ 694.560023] Modules linked in: ip6t_REJECT nf_reject_ipv6 ip6t_rpfilter xt_conntrack ebtable_broute bridge stp llc ebtable_nat ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_raw ip6table_mangle ip6table_security iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_raw iptable_mangle 9p iptable_security ebtable_filter ebtables ip6table_filter ip6_tables snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_seq ppdev snd_seq_device joydev crct10dif_pclmul crc32_pclmul crc32c_intel snd_pcm ghash_clmulni_intel 9pnet_virtio virtio_balloon snd_timer 9pnet parport_pc snd parport i2c_piix4 soundcore nfsd auth_rpcgss nfs_acl lockd grace sunrpc virtio_net virtio_blk virtio_console 8139too qxl drm_kms_helper ttm drm serio_raw 8139cp
[ 694.571554] mii virtio_pci ata_generic virtio_ring virtio pata_acpi
[ 694.572608] CPU: 3 PID: 1200 Comm: test_wipe2 Not tainted 4.13.0-rc3+ #8
[ 694.573778] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.1-1.fc24 04/01/2014
[ 694.574917] task: ffff880137178040 task.stack: ffffc900019d4000
[ 694.575650] RIP: 0010:__delete_from_page_cache+0x344/0x410
[ 694.576409] RSP: 0018:ffffc900019d7a88 EFLAGS: 00010082
[ 694.577238] RAX: 0000000000000021 RBX: ffffea00047d0e00 RCX: 0000000000000006
[ 694.578537] RDX: 0000000000000000 RSI: 0000000000000096 RDI: ffff88023fd0db90
[ 694.579774] RBP: ffffc900019d7ad8 R08: 00000000000882b6 R09: 000000000000028a
[ 694.580754] R10: ffffc900019d7da8 R11: ffffffff8211184d R12: ffffea00047d0e00
[ 694.582040] R13: 0000000000000000 R14: 0000000000000202 R15: ffff8801384439e8
[ 694.583236] FS: 0000000000000000(0000) GS:ffff88023fd00000(0000) knlGS:0000000000000000
[ 694.584607] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 694.585409] CR2: 00007ff77a8da618 CR3: 0000000001e09000 CR4: 00000000001406e0
[ 694.586547] Call Trace:
[ 694.586996] delete_from_page_cache+0x54/0x110
[ 694.587481] truncate_inode_page+0xab/0x120
[ 694.588110] shmem_undo_range+0x498/0xa50
[ 694.588813] ? save_stack_trace+0x1b/0x20
[ 694.589529] ? set_track+0x70/0x140
[ 694.590150] ? init_object+0x69/0xa0
[ 694.590722] ? __inode_wait_for_writeback+0x73/0xe0
[ 694.591525] shmem_truncate_range+0x16/0x40
[ 694.592268] shmem_evict_inode+0xb1/0x190
[ 694.592735] evict+0xbb/0x1c0
[ 694.593147] iput+0x1c0/0x210
[ 694.593497] dentry_unlink_inode+0xb4/0x150
[ 694.593982] __dentry_kill+0xc1/0x150
[ 694.594400] dput+0x1c8/0x1e0
[ 694.594745] __fput+0x172/0x1e0
[ 694.595103] ____fput+0xe/0x10
[ 694.595463] task_work_run+0x80/0xa0
[ 694.595886] do_exit+0x2d6/0xb50
[ 694.596323] ? __do_page_fault+0x288/0x4a0
[ 694.596818] do_group_exit+0x47/0xb0
[ 694.597249] SyS_exit_group+0x14/0x20
[ 694.597682] entry_SYSCALL_64_fastpath+0x1a/0xa5
[ 694.598198] RIP: 0033:0x7ff77a5e78c8
[ 694.598612] RSP: 002b:00007ffc5aece318 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[ 694.599804] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007ff77a5e78c8
[ 694.600609] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[ 694.601424] RBP: 00007ff77a8da618 R08: 00000000000000e7 R09: ffffffffffffff98
[ 694.602224] R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000001
[ 694.603151] R13: 00007ff77a8dbc60 R14: 0000000000000000 R15: 0000000000000000
[ 694.603984] Code: 60 f3 c5 81 e8 2e 7e 03 00 0f 0b 48 c7 c6 60 f3 c5 81 4c 89 e7 e8 1d 7e 03 00 0f 0b 48 c7 c6 00 f4 c5 81 4c 89 e7 e8 0c 7e 03 00 <0f> 0b 48 c7 c6 38 f3 c5 81 4c 89 e7 e8 fb 7d 03 00 0f 0b 48 c7
[ 694.606500] RIP: __delete_from_page_cache+0x344/0x410 RSP: ffffc900019d7a88
[ 694.607426] ---[ end trace 55e6b04ae95d8ce3 ]---
BTW, this was on 4.13.0-rc3 + your patches. Simple test program is below.
--
Mike Kravetz
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#include <errno.h>
#define MADV_WIPEONFORK 18
#define ADDR (void *)(0x0UL)
int main(int argc, char ** argv)
{
unsigned long page_size;
int ret;
void *addr;
char foo;
page_size = sysconf(_SC_PAGE_SIZE);
addr = mmap(ADDR, page_size,
PROT_READ | PROT_WRITE,
MAP_ANONYMOUS|MAP_SHARED, -1, 0);
if (addr == MAP_FAILED) {
perror("mmap");
exit (1);
}
printf("Parent writing 'a' to page\n");
*((char *)addr) = 'a';
ret = madvise(addr, page_size, MADV_WIPEONFORK);
if (ret) {
perror("madvise");
exit (1);
}
if (fork()) {
/* In parent */
sleep(1);
} else {
/* In child */
foo = *((char *)addr);
printf("child read '%c' from page\n", foo);
}
return ret;
}