On 7/14/17 5:55 PM, Jerome Glisse wrote:

> ...
>
> Cheers,
> JA(C)rA'me

Hi Jerome,

I think I just found a couple of new issues, now related to fork/execve.

1) With a fork() followed by execve(), the child process makes a copy of 
the parent mm_struct object, including the "hmm" pointer. Later on, an 
execve() syscall in the child process frees the old mm_struct, and 
destroys the "hmm" object - which apparently it shouldn't do, because 
the "hmm" object is shared between the parent and child processes:

(gdb) bt
#0  hmm_mm_destroy (mm=0xffff88080757aa40) at mm/hmm.c:134
#1  0xffffffff81058567 in __mmdrop (mm=0xffff88080757aa40) at 
kernel/fork.c:889
#2  0xffffffff8105904f in mmdrop (mm=<optimized out>) at 
./include/linux/sched/mm.h:42
#3  __mmput (mm=<optimized out>) at kernel/fork.c:916
#4  mmput (mm=0xffff88080757aa40) at kernel/fork.c:927
#5  0xffffffff811c5a68 in exec_mmap (mm=<optimized out>) at fs/exec.c:1057
#6  flush_old_exec (bprm=<optimized out>) at fs/exec.c:1284
#7  0xffffffff81214460 in load_elf_binary (bprm=0xffff8808133b1978) at 
fs/binfmt_elf.c:855
#8  0xffffffff811c4fce in search_binary_handler 
(bprm=0xffff88081b40cb78) at fs/exec.c:1625
#9  0xffffffff811c6bbf in exec_binprm (bprm=<optimized out>) at 
fs/exec.c:1667
#10 do_execveat_common (fd=<optimized out>, filename=0xffff88080a101200, 
flags=0x0, argv=..., envp=...) at fs/exec.c:1789
#11 0xffffffff811c6fda in do_execve (__envp=<optimized out>, 
__argv=<optimized out>, filename=<optimized out>) at fs/exec.c:1833
#12 SYSC_execve (envp=<optimized out>, argv=<optimized out>, 
filename=<optimized out>) at fs/exec.c:1914
#13 SyS_execve (filename=<optimized out>, argv=0x7f4e5c2aced0, 
envp=0x7f4e5c2aceb0) at fs/exec.c:1909
#14 0xffffffff810018dd in do_syscall_64 (regs=0xffff88081b40cb78) at 
arch/x86/entry/common.c:284
#15 0xffffffff819e2c06 in entry_SYSCALL_64 () at 
arch/x86/entry/entry_64.S:245

This leads to a sporadic memory corruption in the parent process:

Thread 200 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 3685]
0xffffffff811a3efe in __mmu_notifier_invalidate_range_start 
(mm=0xffff880807579000, start=0x7f4e5c62f000, end=0x7f4e5c66f000) at 
mm/mmu_notifier.c:199
199            if (mn->ops->invalidate_range_start)
(gdb) bt
#0  0xffffffff811a3efe in __mmu_notifier_invalidate_range_start 
(mm=0xffff880807579000, start=0x7f4e5c62f000, end=0x7f4e5c66f000) at 
mm/mmu_notifier.c:199
#1  0xffffffff811ae471 in mmu_notifier_invalidate_range_start 
(end=<optimized out>, start=<optimized out>, mm=<optimized out>) at 
./include/linux/mmu_notifier.h:282
#2  migrate_vma_collect (migrate=0xffffc90003ca3940) at mm/migrate.c:2280
#3  0xffffffff811b04a7 in migrate_vma (ops=<optimized out>, 
vma=0x7f4e5c62f000, start=0x7f4e5c62f000, end=0x7f4e5c66f000, 
src=0xffffc90003ca39d0, dst=0xffffc90003ca39d0, 
private=0xffffc90003ca39c0) at mm/migrate.c:2819
(gdb) p mn->ops
$2 = (const struct mmu_notifier_ops *) 0x6b6b6b6b6b6b6b6b

Please see attached a reproducer (sanity_rmem004_fork.tgz). Use 
"./build.sh; sudo ./kload.sh; ./run.sh" to recreate the issue on your end.


2) A slight modification of the affected application does not use 
fork(). Instead, an execve() call from a parallel thread replaces the 
original process. This is a particularly interesting case, because at 
that point the process is busy migrating pages to/from device.

Here's what happens:

0xffffffff811b9879 in commit_charge (page=<optimized out>, 
lrucare=<optimized out>, memcg=<optimized out>) at mm/memcontrol.c:2060
2060        VM_BUG_ON_PAGE(page->mem_cgroup, page);
(gdb) bt
#0  0xffffffff811b9879 in commit_charge (page=<optimized out>, 
lrucare=<optimized out>, memcg=<optimized out>) at mm/memcontrol.c:2060
#1  0xffffffff811b93d6 in commit_charge (lrucare=<optimized out>, 
memcg=<optimized out>, page=<optimized out>) at 
./include/linux/page-flags.h:149
#2  mem_cgroup_commit_charge (page=0xffff88081b68cb70, 
memcg=0xffff88081b051548, lrucare=<optimized out>, compound=<optimized 
out>) at mm/memcontrol.c:5468
#3  0xffffffff811b10d4 in migrate_vma_insert_page (migrate=<optimized 
out>, dst=<optimized out>, src=<optimized out>, page=<optimized out>, 
addr=<optimized out>) at mm/migrate.c:2605
#4  migrate_vma_pages (migrate=<optimized out>) at mm/migrate.c:2647
#5  migrate_vma (ops=<optimized out>, vma=<optimized out>, 
start=<optimized out>, end=<optimized out>, src=<optimized out>, 
dst=<optimized out>, private=0xffffc900037439c0) at mm/migrate.c:2844


Please find another reproducer attached (sanity_rmem004_execve.tgz) for 
this issue.

Thanks!

-- 
Evgeny Baskakov
NVIDIA