Re: [PATCH v10 2/9] fs: introduce kernel_pread_file* support

[Scott Branden <scott.branden@broadcom.com>]

Hi Kees,

one more comment below.

On 2020-07-07 9:01 p.m., Scott Branden wrote:
>
>
> On 2020-07-07 4:56 p.m., Kees Cook wrote:
>> On Mon, Jul 06, 2020 at 04:23:02PM -0700, Scott Branden wrote:
>>> Add kernel_pread_file* support to kernel to allow for partial read
>>> of files with an offset into the file.
>>>
>>> Signed-off-by: Scott Branden <scott.branden@broadcom.com>
>>> ---
>>>   fs/exec.c                        | 93 
>>> ++++++++++++++++++++++++--------
>>>   include/linux/kernel_read_file.h | 17 ++++++
>>>   2 files changed, 87 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/fs/exec.c b/fs/exec.c
>>> index 4ea87db5e4d5..e6a8a65f7478 100644
>>> --- a/fs/exec.c
>>> +++ b/fs/exec.c
>>> @@ -928,10 +928,14 @@ struct file *open_exec(const char *name)
>>>   }
>>>   EXPORT_SYMBOL(open_exec);
>>>   -int kernel_read_file(struct file *file, void **buf, loff_t *size,
>>> -             loff_t max_size, enum kernel_read_file_id id)
>>> -{
>>> -    loff_t i_size, pos;
>>> +int kernel_pread_file(struct file *file, void **buf, loff_t *size,
>>> +              loff_t max_size, loff_t pos,
>>> +              enum kernel_read_file_id id)
>>> +{
>>> +    loff_t alloc_size;
>>> +    loff_t buf_pos;
>>> +    loff_t read_end;
>>> +    loff_t i_size;
>>>       ssize_t bytes = 0;
>>>       int ret;
>>>   @@ -951,21 +955,32 @@ int kernel_read_file(struct file *file, void 
>>> **buf, loff_t *size,
>>>           ret = -EINVAL;
>>>           goto out;
>>>       }
>>> -    if (i_size > SIZE_MAX || (max_size > 0 && i_size > max_size)) {
>>> +
>>> +    /* Default read to end of file */
>>> +    read_end = i_size;
>>> +
>>> +    /* Allow reading partial portion of file */
>>> +    if ((id == READING_FIRMWARE_PARTIAL_READ) &&
>>> +        (i_size > (pos + max_size)))
>>> +        read_end = pos + max_size;
>> There's no need to involve "id" here. There are other signals about
>> what's happening (i.e. pos != 0, max_size != i_size, etc).
> There are other signals other than the fact that kernel_read_file 
> requires
> the entire file to be read while kernel_pread_file allows partial 
> files to be read.
> So if you do a pread at pos = 0 you need another key to indicate it is 
> "ok" if max_size < i_size.
> If id == READING_FIRMWARE_PARTIAL_READ is removed (and we want to 
> share 99% of the code
> between kernel_read_file and kernel_pread_file then I need to add 
> another parameter to a common function
> called between these functions.  And adding another parameter was 
> rejected previously in the review as a "swiss army knife approach" by 
> another reviewer.  I am happy to add it back in because it is 
> necessary to share code and differentiate whether we are performing a 
> partial read or not.
>>
>>> +
>>> +    alloc_size = read_end - pos;
>>> +    if (i_size > SIZE_MAX || (max_size > 0 && alloc_size > 
>>> max_size)) {
>>>           ret = -EFBIG;
>>>           goto out;
>>>       }
>>>   -    if (id != READING_FIRMWARE_PREALLOC_BUFFER)
>>> -        *buf = vmalloc(i_size);
>>> +    if ((id != READING_FIRMWARE_PARTIAL_READ) &&
>>> +        (id != READING_FIRMWARE_PREALLOC_BUFFER))
>>> +        *buf = vmalloc(alloc_size);
>>>       if (!*buf) {
>>>           ret = -ENOMEM;
>>>           goto out;
>>>       }
>> The id usage here was a mistake in upstream, and the series I sent is
>> trying to clean that up.
> I see that cleanup and it works fine with the pread.  Other than I 
> need some sort of key to share code and indicate whether it is "ok" to 
> do a partial read of the file or not.
>>
>> Greg, it seems this series is going to end up in your tree due to it
>> being drivers/misc? I guess I need to direct my series to Greg then, but
>> get LSM folks Acks.
>>
>>>   -    pos = 0;
>>> -    while (pos < i_size) {
>>> -        bytes = kernel_read(file, *buf + pos, i_size - pos, &pos);
>>> +    buf_pos = 0;
>>> +    while (pos < read_end) {
>>> +        bytes = kernel_read(file, *buf + buf_pos, read_end - pos, 
>>> &pos);
>>>           if (bytes < 0) {
>>>               ret = bytes;
>>>               goto out_free;
>>> @@ -973,20 +988,23 @@ int kernel_read_file(struct file *file, void 
>>> **buf, loff_t *size,
>>>             if (bytes == 0)
>>>               break;
>>> +
>>> +        buf_pos += bytes;
>>>       }
>>>   -    if (pos != i_size) {
>>> +    if (pos != read_end) {
>>>           ret = -EIO;
>>>           goto out_free;
>>>       }
>>>   -    ret = security_kernel_post_read_file(file, *buf, i_size, id);
>>> +    ret = security_kernel_post_read_file(file, *buf, alloc_size, id);
>>>       if (!ret)
>>>           *size = pos;
>> This call cannot be inside kernel_pread_file(): any future LSMs will see
>> a moving window of contents, etc. It'll need to be in kernel_read_file()
>> proper.
> If IMA still passes (after testing my next patch series with your 
> changes and my modifications)
> I will need some more help here.
>>
>>>     out_free:
>>>       if (ret < 0) {
>>> -        if (id != READING_FIRMWARE_PREALLOC_BUFFER) {
>>> +        if ((id != READING_FIRMWARE_PARTIAL_READ) &&
>>> +            (id != READING_FIRMWARE_PREALLOC_BUFFER)) {
>>>               vfree(*buf);
>>>               *buf = NULL;
>>>           }
>>> @@ -996,10 +1014,18 @@ int kernel_read_file(struct file *file, void 
>>> **buf, loff_t *size,
>>>       allow_write_access(file);
>>>       return ret;
>>>   }
>>> +
>>> +int kernel_read_file(struct file *file, void **buf, loff_t *size,
>>> +             loff_t max_size, enum kernel_read_file_id id)
>>> +{
>>> +    return kernel_pread_file(file, buf, size, max_size, 0, id);
>>> +}
>>>   EXPORT_SYMBOL_GPL(kernel_read_file);
>>>   -int kernel_read_file_from_path(const char *path, void **buf, 
>>> loff_t *size,
>>> -                   loff_t max_size, enum kernel_read_file_id id)
>>> +int kernel_pread_file_from_path(const char *path, void **buf,
>>> +                loff_t *size,
>>> +                loff_t max_size, loff_t pos,
>>> +                enum kernel_read_file_id id)
>>>   {
>>>       struct file *file;
>>>       int ret;
>>> @@ -1011,15 +1037,22 @@ int kernel_read_file_from_path(const char 
>>> *path, void **buf, loff_t *size,
>>>       if (IS_ERR(file))
>>>           return PTR_ERR(file);
>>>   -    ret = kernel_read_file(file, buf, size, max_size, id);
>>> +    ret = kernel_pread_file(file, buf, size, max_size, pos, id);
>>>       fput(file);
>>>       return ret;
>>>   }
>>> +
>>> +int kernel_read_file_from_path(const char *path, void **buf, loff_t 
>>> *size,
>>> +                   loff_t max_size, enum kernel_read_file_id id)
>>> +{
>>> +    return kernel_pread_file_from_path(path, buf, size, max_size, 
>>> 0, id);
>>> +}
>>>   EXPORT_SYMBOL_GPL(kernel_read_file_from_path);
>>>   -int kernel_read_file_from_path_initns(const char *path, void **buf,
>>> -                      loff_t *size, loff_t max_size,
>>> -                      enum kernel_read_file_id id)
>>> +int kernel_pread_file_from_path_initns(const char *path, void **buf,
>>> +                       loff_t *size,
>>> +                       loff_t max_size, loff_t pos,
>>> +                       enum kernel_read_file_id id)
>>>   {
>>>       struct file *file;
>>>       struct path root;
>>> @@ -1037,14 +1070,22 @@ int kernel_read_file_from_path_initns(const 
>>> char *path, void **buf,
>>>       if (IS_ERR(file))
>>>           return PTR_ERR(file);
>>>   -    ret = kernel_read_file(file, buf, size, max_size, id);
>>> +    ret = kernel_pread_file(file, buf, size, max_size, pos, id);
>>>       fput(file);
>>>       return ret;
>>>   }
>>> +
>>> +int kernel_read_file_from_path_initns(const char *path, void **buf,
>>> +                      loff_t *size, loff_t max_size,
>>> +                      enum kernel_read_file_id id)
>>> +{
>>> +    return kernel_pread_file_from_path_initns(path, buf, size, 
>>> max_size, 0, id);
>>> +}
>>>   EXPORT_SYMBOL_GPL(kernel_read_file_from_path_initns);
>>>   -int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, 
>>> loff_t max_size,
>>> -                 enum kernel_read_file_id id)
>>> +int kernel_pread_file_from_fd(int fd, void **buf, loff_t *size,
>>> +                  loff_t max_size, loff_t pos,
>>> +                  enum kernel_read_file_id id)
>>>   {
>>>       struct fd f = fdget(fd);
>>>       int ret = -EBADF;
>>> @@ -1052,11 +1093,17 @@ int kernel_read_file_from_fd(int fd, void 
>>> **buf, loff_t *size, loff_t max_size,
>>>       if (!f.file)
>>>           goto out;
>>>   -    ret = kernel_read_file(f.file, buf, size, max_size, id);
>>> +    ret = kernel_pread_file(f.file, buf, size, max_size, pos, id);
>>>   out:
>>>       fdput(f);
>>>       return ret;
>>>   }
>>> +
>>> +int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, 
>>> loff_t max_size,
>>> +                 enum kernel_read_file_id id)
>>> +{
>>> +    return kernel_pread_file_from_fd(fd, buf, size, max_size, 0, id);
>>> +}
>>>   EXPORT_SYMBOL_GPL(kernel_read_file_from_fd);
>> For each of these execution path, the mapping to LSM hooks is:
>>
>> - all path must call security_kernel_read_file(file, id) before reading
>>    (this appears to be fine as-is in your series).
>>
>> - anything doing a "full" read needs to call
>>    security_kernel_post_read_file() with the file and full buffer, size,
>>    etc (so all the kernel_read_file*() paths). I imagine instead of
>>    adding 3 copy/pasted versions of this, it may be possible to refactor
>>    the helpers into a single core "full" caller that takes struct file,
>>    or doing some logic in kernel_pread_file() that notices it has the
>>    entire file in the buffer and doing the call then.
>>    As an example of what I mean about doing the call, here's how I might
>>    imagine it for one of the paths if it took struct file:
>>
>> int kernel_read_file_from_file(struct file *file, void **buf, loff_t 
>> *size,
>>                    loff_t max_size, enum kernel_read_file_id id)
>> {
>>     int ret;
>>
>>     ret = kernel_pread_file_from_file(file, buf, size, max_size, 0, id);
>>     if (ret)
>>         return ret;
>>     return security_kernel_post_read_file(file, buf, *size, id);
>> }
>>
>>>     #if defined(CONFIG_HAVE_AOUT) || defined(CONFIG_BINFMT_FLAT) || \
>>> diff --git a/include/linux/kernel_read_file.h 
>>> b/include/linux/kernel_read_file.h
>>> index 53f5ca41519a..f061ccb8d0b4 100644
>>> --- a/include/linux/kernel_read_file.h
>>> +++ b/include/linux/kernel_read_file.h
>>> @@ -8,6 +8,7 @@
>>>   #define __kernel_read_file_id(id) \
>>>       id(UNKNOWN, unknown)        \
>>>       id(FIRMWARE, firmware)        \
>>> +    id(FIRMWARE_PARTIAL_READ, firmware)    \
>>>       id(FIRMWARE_PREALLOC_BUFFER, firmware)    \
>>>       id(FIRMWARE_EFI_EMBEDDED, firmware)    \
>> And again, sorry that this was in here as a misleading example.
>>
>>>       id(MODULE, kernel-module)        \
>>> @@ -36,15 +37,31 @@ static inline const char 
>>> *kernel_read_file_id_str(enum kernel_read_file_id id)
>>>       return kernel_read_file_str[id];
>>>   }
>>>   +int kernel_pread_file(struct file *file,
>>> +              void **buf, loff_t *size, loff_t pos,
>>> +              loff_t max_size,
>>> +              enum kernel_read_file_id id);
>>>   int kernel_read_file(struct file *file,
>>>                void **buf, loff_t *size, loff_t max_size,
>>>                enum kernel_read_file_id id);
>>> +int kernel_pread_file_from_path(const char *path,
>>> +                void **buf, loff_t *size, loff_t pos,
>>> +                loff_t max_size,
>>> +                enum kernel_read_file_id id);
>>>   int kernel_read_file_from_path(const char *path,
>>>                      void **buf, loff_t *size, loff_t max_size,
>>>                      enum kernel_read_file_id id);
>>> +int kernel_pread_file_from_path_initns(const char *path,
>>> +                       void **buf, loff_t *size, loff_t pos,
>>> +                       loff_t max_size,
>>> +                       enum kernel_read_file_id id);
>>>   int kernel_read_file_from_path_initns(const char *path,
>>>                         void **buf, loff_t *size, loff_t max_size,
>>>                         enum kernel_read_file_id id);
>>> +int kernel_pread_file_from_fd(int fd,
>>> +                  void **buf, loff_t *size, loff_t pos,
>>> +                  loff_t max_size,
>>> +                  enum kernel_read_file_id id);
>>>   int kernel_read_file_from_fd(int fd,
>>>                    void **buf, loff_t *size, loff_t max_size,
>>>                    enum kernel_read_file_id id);
>> I remain concerned that adding these helpers will lead a poor
>> interaction with LSMs, but I guess I get to hold my tongue. :)
I only need kernel_pread_file and kernel_pread_file_from_path_initns.  
kernel_pread_file_from_fd and kernel_pread_file_from_path were only 
added for completeness.
And are really only helper functions called by their kernel_read_file* 
counterparts at this time.  So they can be removed from this patch if 
that helps?
> We could add pread functions that are "unsafe" in nature instead then?
> As I certainly do not need any integrity checks on the file for my 
> driver.  The real check is done by the card the data is loaded to 
> whether is passes the linux security checks or not.
> And then, if someone does want to do something "safe" with preads 
> another kernel_read_file_securelock/unlock could be added for those 
> that need security for their partial reads?
>>
>