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Kernel: Fix subtle race condition in sys$write implementation

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There is a slight race condition in our implementation of write().
We call File::can_write() before attempting to write to it (blocking if
it returns false). If it returns true, we assume that we can write to
the file, and our code assumes that File::write() cannot possibly fail
by being blocked. There is, however, the rare case where another process
writes to the file and prevents further writes in between the call to
Files::can_write() and File::write() in the first process. This would
result in the first process calling File::write() when it cannot be
written to.

We fix this by adding a mechanism for File::can_write() to signal that
it was blocked, making it the responsibilty of File::write() to check
whether it can write and then finally making sys$write() check if the
write failed due to it being blocked.
This commit is contained in:
Sahan Fernando 2021-05-18 21:49:34 +10:00 • committed by Andreas Kling
parent 208cfcb0a5
commit d0f314b23c
3 changed files with 15 additions and 14 deletions
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8
Kernel/Devices/SerialDevice.cpp
View file

@ -31,6 +31,7 @@ KResultOr<size_t> SerialDevice::read(FileDescription&, u64, UserOrKernelBuffer&
if (!size)
return 0;
ScopedSpinLock lock(m_serial_lock);
if (!(get_line_status() & DataReady))
return 0;
@ -46,13 +47,14 @@ bool SerialDevice::can_write(const FileDescription&, size_t) const
return (get_line_status() & EmptyTransmitterHoldingRegister) != 0;
}
KResultOr<size_t> SerialDevice::write(FileDescription&, u64, const UserOrKernelBuffer& buffer, size_t size)
KResultOr<size_t> SerialDevice::write(FileDescription& description, u64, const UserOrKernelBuffer& buffer, size_t size)
{
if (!size)
return 0;
if (!(get_line_status() & EmptyTransmitterHoldingRegister))
return 0;
ScopedSpinLock lock(m_serial_lock);
if (!can_write(description, size))
return EAGAIN;
return buffer.read_buffered<128>(size, [&](u8 const* data, size_t data_size) {
for (size_t i = 0; i < data_size; i++)

1
Kernel/Devices/SerialDevice.h
View file

@ -135,6 +135,7 @@ private:
bool m_break_enable { false };
u8 m_modem_control { 0 };
bool m_last_put_char_was_carriage_return { false };
SpinLock<u8> m_serial_lock;
};
}

18
Kernel/Syscalls/write.cpp
View file

@ -60,10 +60,6 @@ KResultOr<ssize_t> Process::sys$writev(int fd, Userspace<const struct iovec*> io
KResultOr<ssize_t> Process::do_write(FileDescription& description, const UserOrKernelBuffer& data, size_t data_size)
{
ssize_t total_nwritten = 0;
if (!description.is_blocking()) {
if (!description.can_write())
return EAGAIN;
}
if (description.should_append() && description.file().is_seekable()) {
auto seek_result = description.seek(0, SEEK_END);
@ -72,11 +68,12 @@ KResultOr<ssize_t> Process::do_write(FileDescription& description, const UserOrK
}
while ((size_t)total_nwritten < data_size) {
if (!description.can_write()) {
while (!description.can_write()) {
if (!description.is_blocking()) {
// Short write: We can no longer write to this non-blocking description.
VERIFY(total_nwritten > 0);
if (total_nwritten > 0)
return total_nwritten;
else
return EAGAIN;
}
auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
if (Thread::current()->block<Thread::WriteBlocker>({}, description, unblock_flags).was_interrupted()) {
@ -87,12 +84,13 @@ KResultOr<ssize_t> Process::do_write(FileDescription& description, const UserOrK
}
auto nwritten_or_error = description.write(data.offset(total_nwritten), data_size - total_nwritten);
if (nwritten_or_error.is_error()) {
if (total_nwritten)
if (total_nwritten > 0)
return total_nwritten;
if (nwritten_or_error.error() == EAGAIN)
continue;
return nwritten_or_error.error();
}
if (nwritten_or_error.value() == 0)
break;
VERIFY(nwritten_or_error.value() > 0);
total_nwritten += nwritten_or_error.value();
}
return total_nwritten;