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Kernel: Implement the same symlink protection as Linux

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Path resolution will now refuse to follow symlinks in some cases where
you don't own the symlink, or when it's in a sticky world-writable
directory and the link has a different owner than the directory.

The point of all this is to prevent classic TOCTOU bugs in /tmp etc.

Fixes #4934
This commit is contained in:
Andreas Kling 2021-01-19 18:12:09 +01:00
parent 9681e3eca0
commit 8601108e21
1 changed files with 19 additions and 0 deletions
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19
Kernel/FileSystem/VirtualFileSystem.cpp
View file

@ -952,6 +952,21 @@ KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& ba
return custody; return custody;
} }
static bool safe_to_follow_symlink(const Inode& inode, const InodeMetadata& parent_metadata)
{
auto metadata = inode.metadata();
if (Process::current()->euid() == metadata.uid)
return true;
if (!(parent_metadata.is_sticky() && parent_metadata.mode & S_IWOTH))
return true;
if (metadata.uid == parent_metadata.uid)
return true;
return false;
}
KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path_without_veil(StringView path, Custody& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path_without_veil(StringView path, Custody& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level)
{ {
if (symlink_recursion_level >= symlink_recursion_limit) if (symlink_recursion_level >= symlink_recursion_limit)
@ -1017,6 +1032,10 @@ KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path_without_veil(StringView path
if (options & O_NOFOLLOW_NOERROR) if (options & O_NOFOLLOW_NOERROR)
break; break;
} }
if (!safe_to_follow_symlink(*child_inode, parent_metadata))
return KResult(-EACCES);
auto symlink_target = child_inode->resolve_as_link(parent, out_parent, options, symlink_recursion_level + 1); auto symlink_target = child_inode->resolve_as_link(parent, out_parent, options, symlink_recursion_level + 1);
if (symlink_target.is_error() || !have_more_parts) if (symlink_target.is_error() || !have_more_parts)
return symlink_target; return symlink_target;