[pbs-devel] [PATCH proxmox-backup] fix #4380: check permissions before entering directory

[Gabriel Goller]

Thanks for the feedback, submitted a new patch (v3).

On 8/8/23 12:25, Wolfgang Bumiller wrote:
> NAK.
>
> You can never assume that the permission bits apply to your user, and if
> they do, there can be ACLs and security-module rules (Apparmor, SELinux,
> ...) involved and a lot more.
> Permission checks are much more complicated, and checking them manually
> is *always* wrong. Handling an `EPERM`/`EACCESS` error is the only
> correct way.
> (The closest you'd get would be with the `eaccess()` syscall, but that's
> also unnecessarily racy, and an extra call for something you just don't
> need...)
>
> See my reply to the other patch about how I'd tackle this.
>
> Anyway, further code comments down below.
>
> On Fri, Aug 04, 2023 at 12:02:25PM +0200, Gabriel Goller wrote:
>> When creating a backup, we now check if we have the correct permissions
>> (r,x) before entering a directory. This is mainly to prevent stat() from
>> failing with EACCESS errors. We also check if the directory contains
>> non-excluded files and warn the user.
>>
>> Signed-off-by: Gabriel Goller <g.goller at proxmox.com>
>> ---
>>   pbs-client/src/pxar/create.rs | 47 +++++++++++++++++++++++++++++++++--
>>   1 file changed, 45 insertions(+), 2 deletions(-)
>>
>> diff --git a/pbs-client/src/pxar/create.rs b/pbs-client/src/pxar/create.rs
>> index 2577cf98..f2333284 100644
>> --- a/pbs-client/src/pxar/create.rs
>> +++ b/pbs-client/src/pxar/create.rs
>> @@ -638,7 +638,7 @@ impl Archiver {
>>       async fn add_directory<T: SeqWrite + Send>(
>>           &mut self,
>>           encoder: &mut Encoder<'_, T>,
>> -        dir: Dir,
>> +        mut dir: Dir,
>>           dir_name: &CStr,
>>           metadata: &Metadata,
>>           stat: &FileStat,
>> @@ -663,9 +663,52 @@ impl Archiver {
>>                   skip_contents = !set.contains(&stat.st_dev);
>>               }
>>           }
>> +        if skip_contents {
>> +            log::warn!("Skipping mount point: {:?}", self.path);
>> +        }
>> +
>> +        let mode = nix::sys::stat::Mode::from_bits_truncate(stat.st_mode);
>> +        // if we have read and write permissions on the folder
> ^ wrong comment?
>
>> +        if (!mode.contains(Mode::S_IRUSR) || !mode.contains(Mode::S_IXUSR))
> ^ if you look at the bitflags docs' description of `.contains()` vs
> `.intersects()` you'll see that `.contains()` requires all the bits in
> question to be set.
> You're asking "(does not contain A) or (does not contain B)"
> => which means: "not (both are set)"
> => `!mode.contains(Mode::S_IRUSR | Mode::S_IXUSR)` should be a shorter
> version of the same, no? :-)
>
>> +            && skip_contents == false
>> +        {
>> +            skip_contents = true;
>> +            let mut contains_non_excluded_files = false;
>> +            if mode.contains(Mode::S_IRUSR) {
>> +                // check if all children are excluded
>> +                for file in dir.iter() {
>> +                    let file = file?;
>> +
>> +                    let file_name = file.file_name().to_owned();
> So this bit is copied - but the original could use some cleanup ;-)
> `to_owned()` allocates and is not necessary for the `.to_bytes()` call
> and we don't actually need it owned anywhere up until the end
>
>> +                    let file_name_bytes = file_name.to_bytes();
>> +                    if file_name_bytes == b"." || file_name_bytes == b".." {
>> +                        continue;
>> +                    }
>> +                    let os_file_name = OsStr::from_bytes(file_name_bytes);
>> +                    assert_single_path_component(os_file_name)?;
>> +                    let full_path = self.path.join(os_file_name);
>> +                    let match_path = PathBuf::from("/").join(full_path.clone());
>> +                    if self
>> +                        .patterns
>> +                        .matches(match_path.as_os_str().as_bytes(), Some(stat.st_mode))
>> +                        != Some(MatchType::Exclude)
>> +                    {
>> +                        contains_non_excluded_files = true;
>> +                        break;
>> +                    }
>> +                }
>> +            }
>> +            if contains_non_excluded_files {
>> +                log::warn!(
>> +                    "Skipping directory: {:?}, access denied (contains non-excluded files)",
> ^ that is a weird error message which sounds like the existence of files
> is the reason the access is denied ;-)
>
>> +                    self.path
>> +                );
>> +            } else {
>> +                log::warn!("Skipping directory: {:?}, access denied", self.path);
>> +            }
>> +        }
>>   
>>           let result = if skip_contents {
>> -            log::info!("skipping mount point: {:?}", self.path);
>>               Ok(())
>>           } else {
>>               self.archive_dir_contents(&mut encoder, dir, false).await
>> -- 
>> 2.39.2