Re: [PATCH V2] proc: fix ubsan warning in mem_lseek

[yangerkun <yangerkun@huawei.com>]

On 2019/9/26 3:20, Alexey Dobriyan wrote:
> On Tue, Sep 17, 2019 at 02:09:38PM +0800, yangerkun wrote:
>> Ping again...
>>
>> On 2019/9/6 13:48, yangerkun wrote:
>>> Ping.
>>>
>>> On 2019/9/2 14:57, yangerkun wrote:
>>>> UBSAN has reported a overflow with mem_lseek. And it's fine with
>>>> mem_open set file mode with FMODE_UNSIGNED_OFFSET(memory_lseek).
>>>> However, another file use mem_lseek do lseek can have not
>>>> FMODE_UNSIGNED_OFFSET(proc_kpagecount_operations/proc_pagemap_operations),
> 
> Why those files can't have FMODE_UNSIGNED_OFFSET?
> All files have unsigned offsets by definition, it is just lseek(SEEK_SET)
> reusing signed type is silly.

I am not sure i understand what you say completely...

What i have seen is that every filesystem should set 
FMODE_UNSIGNED_OFFSET in its own 'open' operations(like mem_open) once 
they should use negative loff_t.

> 
>>>> fix it by checking overflow and FMODE_UNSIGNED_OFFSET.
>>>>
>>>> ==================================================================
>>>> UBSAN: Undefined behaviour in ../fs/proc/base.c:941:15
>>>> signed integer overflow:
>>>> 4611686018427387904 + 4611686018427387904 cannot be represented in
>>>> type 'long long int'
>>>> CPU: 4 PID: 4762 Comm: syz-executor.1 Not tainted 4.4.189 #3
>>>> Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
>>>> Call trace:
>>>> [<ffffff90080a5f28>] dump_backtrace+0x0/0x590
>>>> arch/arm64/kernel/traps.c:91
>>>> [<ffffff90080a64f0>] show_stack+0x38/0x60 arch/arm64/kernel/traps.c:234
>>>> [<ffffff9008986a34>] __dump_stack lib/dump_stack.c:15 [inline]
>>>> [<ffffff9008986a34>] dump_stack+0x128/0x184 lib/dump_stack.c:51
>>>> [<ffffff9008a2d120>] ubsan_epilogue+0x34/0x9c lib/ubsan.c:166
>>>> [<ffffff9008a2d8b8>] handle_overflow+0x228/0x280 lib/ubsan.c:197
>>>> [<ffffff9008a2da2c>] __ubsan_handle_add_overflow+0x4c/0x68
>>>> lib/ubsan.c:204
>>>> [<ffffff900862b9f4>] mem_lseek+0x12c/0x130 fs/proc/base.c:941
>>>> [<ffffff90084ef78c>] vfs_llseek fs/read_write.c:260 [inline]
>>>> [<ffffff90084ef78c>] SYSC_lseek fs/read_write.c:285 [inline]
>>>> [<ffffff90084ef78c>] SyS_lseek+0x164/0x1f0 fs/read_write.c:276
>>>> [<ffffff9008093c80>] el0_svc_naked+0x30/0x34
>>>> ==================================================================
>>>>
>>>> Signed-off-by: yangerkun <yangerkun@huawei.com>
>>>> ---
>>>>    fs/proc/base.c     | 32 ++++++++++++++++++++++++--------
>>>>    fs/read_write.c    |  5 -----
>>>>    include/linux/fs.h |  5 +++++
>>>>    3 files changed, 29 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>>> index ebea950..a6c701b 100644
>>>> --- a/fs/proc/base.c
>>>> +++ b/fs/proc/base.c
>>>> @@ -882,18 +882,34 @@ static ssize_t mem_write(struct file *file,
>>>> const char __user *buf,
>>>>      loff_t mem_lseek(struct file *file, loff_t offset, int orig)
>>>>    {
>>>> +    loff_t ret = 0;
>>>> +
>>>> +    spin_lock(&file->f_lock);
>>>>        switch (orig) {
>>>> -    case 0:
>>>> -        file->f_pos = offset;
>>>> -        break;
>>>> -    case 1:
>>>> -        file->f_pos += offset;
>>>> +    case SEEK_CUR:
>>>> +        offset += file->f_pos;
>>>> +        /* fall through */
>>>> +    case SEEK_SET:
>>>> +        /* to avoid userland mistaking f_pos=-9 as -EBADF=-9 */
>>>> +        if ((unsigned long long)offset >= -MAX_ERRNO)
>>>> +            ret = -EOVERFLOW;
>>>>            break;
>>>>        default:
>>>> -        return -EINVAL;
>>>> +        ret = -EINVAL;
>>>> +    }
>>>> +
>>>> +    if (!ret) {
>>>> +        if (offset < 0 && !(unsigned_offsets(file))) {
>>>> +            ret = -EINVAL;
>>>> +        } else {
>>>> +            file->f_pos = offset;
>>>> +            ret = file->f_pos;
>>>> +            force_successful_syscall_return();
>>>> +        }
>>>>        }
>>>> -    force_successful_syscall_return();
>>>> -    return file->f_pos;
>>>> +
>>>> +    spin_unlock(&file->f_lock);
>>>> +    return ret;
>>>>    }
>>>>      static int mem_release(struct inode *inode, struct file *file)
>>>> diff --git a/fs/read_write.c b/fs/read_write.c
>>>> index 5bbf587..961966e 100644
>>>> --- a/fs/read_write.c
>>>> +++ b/fs/read_write.c
>>>> @@ -34,11 +34,6 @@ const struct file_operations generic_ro_fops = {
>>>>      EXPORT_SYMBOL(generic_ro_fops);
>>>>    -static inline bool unsigned_offsets(struct file *file)
>>>> -{
>>>> -    return file->f_mode & FMODE_UNSIGNED_OFFSET;
>>>> -}
>>>> -
>>>>    /**
>>>>     * vfs_setpos - update the file offset for lseek
>>>>     * @file:    file structure in question
>>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>>> index 997a530..e5edbc9 100644
>>>> --- a/include/linux/fs.h
>>>> +++ b/include/linux/fs.h
>>>> @@ -3074,6 +3074,11 @@ extern void
>>>>    file_ra_state_init(struct file_ra_state *ra, struct address_space
>>>> *mapping);
>>>>    extern loff_t noop_llseek(struct file *file, loff_t offset, int
>>>> whence);
>>>>    extern loff_t no_llseek(struct file *file, loff_t offset, int whence);
>>>> +static inline bool unsigned_offsets(struct file *file)
>>>> +{
>>>> +    return file->f_mode & FMODE_UNSIGNED_OFFSET;
>>>> +}
>>>> +
>>>>    extern loff_t vfs_setpos(struct file *file, loff_t offset, loff_t
>>>> maxsize);
>>>>    extern loff_t generic_file_llseek(struct file *file, loff_t offset,
>>>> int whence);
>>>>    extern loff_t generic_file_llseek_size(struct file *file, loff_t
>>>> offset,
>>
> 
> .
>