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Kernel: Make file-backed memory regions remember description permissions

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This allows sys$mprotect() to honor the original readable & writable
flags of the open file description as they were at the point we did the
original sys$mmap().

IIUC, this is what Dr. POSIX wants us to do:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/mprotect.html

Also, remove the bogus and racy "W^X" checking we did against mappings
based on their current inode metadata. If we want to do this, we can do
it properly. For now, it was not only racy, but also did blocking I/O
while holding a spinlock.
This commit is contained in:
Andreas Kling 2022-08-23 20:18:39 +02:00
parent 30861daa93
commit d3e8eb5918
7 changed files with 33 additions and 32 deletions
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33
Kernel/Syscalls/mmap.cpp
View file

@ -103,25 +103,18 @@ ErrorOr<void> Process::validate_mmap_prot(int prot, bool map_stack, bool map_ano
return {};
}
ErrorOr<void> Process::validate_inode_mmap_prot(int prot, Inode const& inode, bool map_shared) const
ErrorOr<void> Process::validate_inode_mmap_prot(int prot, bool readable_description, bool description_writable, bool map_shared) const
{
auto credentials = this->credentials();
auto metadata = inode.metadata();
if ((prot & PROT_READ) && !metadata.may_read(credentials))
if ((prot & PROT_READ) && !readable_description)
return EACCES;
if (map_shared) {
// FIXME: What about readonly filesystem mounts? We cannot make a
// decision here without knowing the mount flags, so we would need to
// keep a Custody or something from mmap time.
if ((prot & PROT_WRITE) && !metadata.may_write(credentials))
if ((prot & PROT_WRITE) && !description_writable)
return EACCES;
if (auto shared_vmobject = inode.shared_vmobject()) {
if ((prot & PROT_EXEC) && shared_vmobject->writable_mappings())
return EACCES;
if ((prot & PROT_WRITE) && shared_vmobject->executable_mappings())
return EACCES;
}
}
return {};
}
@ -227,7 +220,7 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<Syscall::SC_mmap_params const*> use
return EACCES;
}
if (description->inode())
TRY(validate_inode_mmap_prot(prot, *description->inode(), map_shared));
TRY(validate_inode_mmap_prot(prot, description->is_readable(), description->is_writable(), map_shared));
vmobject = TRY(description->vmobject_for_mmap(*this, requested_range, used_offset, map_shared));
}
@ -251,7 +244,11 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<Syscall::SC_mmap_params const*> use
if (!region)
return ENOMEM;
region->set_mmap(true);
if (description)
region->set_mmap(true, description->is_readable(), description->is_writable());
else
region->set_mmap(true, false, false);
if (map_shared)
region->set_shared(true);
if (map_stack)
@ -289,7 +286,7 @@ ErrorOr<FlatPtr> Process::sys$mprotect(Userspace<void*> addr, size_t size, int p
if (whole_region->access() == Memory::prot_to_region_access_flags(prot))
return 0;
if (whole_region->vmobject().is_inode())
TRY(validate_inode_mmap_prot(prot, static_cast<Memory::InodeVMObject const&>(whole_region->vmobject()).inode(), whole_region->is_shared()));
TRY(validate_inode_mmap_prot(prot, whole_region->mmapped_from_readable(), whole_region->mmapped_from_writable(), whole_region->is_shared()));
whole_region->set_readable(prot & PROT_READ);
whole_region->set_writable(prot & PROT_WRITE);
whole_region->set_executable(prot & PROT_EXEC);
@ -306,7 +303,7 @@ ErrorOr<FlatPtr> Process::sys$mprotect(Userspace<void*> addr, size_t size, int p
if (old_region->access() == Memory::prot_to_region_access_flags(prot))
return 0;
if (old_region->vmobject().is_inode())
TRY(validate_inode_mmap_prot(prot, static_cast<Memory::InodeVMObject const&>(old_region->vmobject()).inode(), old_region->is_shared()));
TRY(validate_inode_mmap_prot(prot, old_region->mmapped_from_readable(), old_region->mmapped_from_writable(), old_region->is_shared()));
// Remove the old region from our regions tree, since were going to add another region
// with the exact same start address.
@ -339,7 +336,8 @@ ErrorOr<FlatPtr> Process::sys$mprotect(Userspace<void*> addr, size_t size, int p
return EPERM;
TRY(validate_mmap_prot(prot, region->is_stack(), region->vmobject().is_anonymous(), region));
if (region->vmobject().is_inode())
TRY(validate_inode_mmap_prot(prot, static_cast<Memory::InodeVMObject const&>(region->vmobject()).inode(), region->is_shared()));
TRY(validate_inode_mmap_prot(prot, region->mmapped_from_readable(), region->mmapped_from_writable(), region->is_shared()));
full_size_found += region->range().intersect(range_to_mprotect).size();
}
@ -490,11 +488,14 @@ ErrorOr<FlatPtr> Process::sys$mremap(Userspace<Syscall::SC_mremap_params const*>
auto new_vmobject = TRY(Memory::PrivateInodeVMObject::try_create_with_inode(inode));
auto old_name = old_region->take_name();
bool old_region_was_mmapped_from_readable = old_region->mmapped_from_readable();
bool old_region_was_mmapped_from_writable = old_region->mmapped_from_writable();
old_region->unmap();
space->deallocate_region(*old_region);
auto* new_region = TRY(space->allocate_region_with_vmobject(range, move(new_vmobject), old_offset, old_name->view(), old_prot, false));
new_region->set_mmap(true);
new_region->set_mmap(true, old_region_was_mmapped_from_readable, old_region_was_mmapped_from_writable);
return new_region->vaddr().get();
}