Re: [PATCH 2/3] shmem: update memory reservation on truncate

Konstantin Khlebnikov
2014-06-26T11:27:47Z

On Thu, Jun 26, 2014 at 7:53 AM, Hugh Dickins <hughd@google.com> wrote:
> On Wed, 25 Jun 2014, Konstantin Khlebnikov wrote:
>
>> Shared anonymous mapping created without MAP_NORESERVE holds memory
>> reservation for whole range of shmem segment. Usually there is no way to
>> change its size, but /proc/<pid>/map_files/...
>> (available if CONFIG_CHECKPOINT_RESTORE=y) allows to do that.
>>
>> This patch adjust memory reservation in shmem_setattr().
>>
>> Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
>
> Acked-by: Hugh Dickins <hughd@google.com>
>
> Thank you, I knew nothing about this backdoor to shmem objects. Scary.
> Was this really the only problem map_files access leads to? If you
> did not do so already, please try to think through other possibilities.

Ouch, it's still broken. I've fixed only truncate.
write_begin/write_end and fallocate might change i_size too.

>
> I haven't begun, but perhaps it's not so bad. I guess the interaction
> with mremap extension is benign - it's annoyed people in the past that
> the underlying shmem object is not extended, but now here's a way that
> it can be.
>
> (I'll leave it to others comment on 3/3 if they wish.)
>
>>
>> ---
>>
>> exploit:
>>
>> #include <sys/mman.h>
>> #include <unistd.h>
>> #include <stdio.h>
>>
>> int main(int argc, char **argv)
>> {
>> unsigned long addr;
>> char path[100];
>>
>> /* charge 4KiB */
>> addr = (unsigned long)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
>> sprintf(path, "/proc/self/map_files/%lx-%lx", addr, addr + 4096);
>> truncate(path, 1 << 30);
>> /* uncharge 1GiB */
>> }
>> ---
>> mm/shmem.c | 17 +++++++++++++++++
>> 1 file changed, 17 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 0aabcbd..a3c49d6 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -149,6 +149,19 @@ static inline void shmem_unacct_size(unsigned long flags, loff_t size)
>> vm_unacct_memory(VM_ACCT(size));
>> }
>>
>> +static inline int shmem_reacct_size(unsigned long flags,
>> + loff_t oldsize, loff_t newsize)
>> +{
>> + if (!(flags & VM_NORESERVE)) {
>> + if (VM_ACCT(newsize) > VM_ACCT(oldsize))
>> + return security_vm_enough_memory_mm(current->mm,
>> + VM_ACCT(newsize) - VM_ACCT(oldsize));
>> + else if (VM_ACCT(newsize) < VM_ACCT(oldsize))
>> + vm_unacct_memory(VM_ACCT(oldsize) - VM_ACCT(newsize));
>> + }
>> + return 0;
>> +}
>> +
>> /*
>> * ... whereas tmpfs objects are accounted incrementally as
>> * pages are allocated, in order to allow huge sparse files.
>> @@ -543,6 +556,10 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
>> loff_t newsize = attr->ia_size;
>>
>> if (newsize != oldsize) {
>> + error = shmem_reacct_size(SHMEM_I(inode)->flags,
>> + oldsize, newsize);
>> + if (error)
>> + return error;
>> i_size_write(inode, newsize);
>> inode->i_ctime = inode->i_mtime = CURRENT_TIME;
>> }
>>
>>