In uart_throttle() and uart_unthrottle(): if (port->status & mask) { port->ops->throttle/unthrottle(port); mask &= ~port->status; } // Code segment utilizing the mask value to determine UART behavior
In uart_change_line_settings(): uart_port_lock_irq(uport); // Code segment responsible for updating uport->status uart_port_unlock_irq(uport);
In the uart_throttle() and uart_unthrottle() functions, there is a double fetch issue due to concurrent execution with uart_change_line_settings(). In uart_throttle() and uart_unthrottle(), the check if (port->status & mask) is made, followed by mask &= ~port->status, where the relevant bits are cleared. However, port->status may be modified in uart_change_line_settings(). The current implementation does not ensure atomicity in the access and modification of port->status and mask. This can result in mask being updated based on a modified port->status value, leading to improper UART actions.
This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 5.17.
To resolve this double fetch, it is suggested to add a uart_port_lock pair in uart_throttle() and uart_unthrottle(). With this patch applied, our tool no longer reports the bug, with the kernel configuration allyesconfig for x86_64. Due to the absence of the requisite hardware, we are unable to conduct runtime testing of the patch. Therefore, our verification is solely based on code logic analysis.
[1] https://sites.google.com/view/basscheck/
Fixes: 391f93f2ec9f ("serial: core: Rework hw-assisted flow control support") Cc: stable@vger.kernel.org Signed-off-by: Gui-Dong Han 2045gemini@gmail.com --- drivers/tty/serial/serial_core.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 80085b151b34..9d905fdf2843 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -723,11 +723,13 @@ static void uart_throttle(struct tty_struct *tty) mask |= UPSTAT_AUTOXOFF; if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS; - + + uart_port_lock_irq(port); if (port->status & mask) { port->ops->throttle(port); mask &= ~port->status; } + uart_port_unlock_irq(port);
if (mask & UPSTAT_AUTORTS) uart_clear_mctrl(port, TIOCM_RTS); @@ -753,10 +755,12 @@ static void uart_unthrottle(struct tty_struct *tty) if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS;
+ uart_port_lock_irq(port); if (port->status & mask) { port->ops->unthrottle(port); mask &= ~port->status; } + uart_port_unlock_irq(port);
if (mask & UPSTAT_AUTORTS) uart_set_mctrl(port, TIOCM_RTS);
On 2024-01-12, Gui-Dong Han 2045gemini@gmail.com wrote:
In uart_throttle() and uart_unthrottle(): if (port->status & mask) { port->ops->throttle/unthrottle(port); mask &= ~port->status; } // Code segment utilizing the mask value to determine UART behavior
In uart_change_line_settings(): uart_port_lock_irq(uport); // Code segment responsible for updating uport->status uart_port_unlock_irq(uport);
In the uart_throttle() and uart_unthrottle() functions, there is a double fetch issue due to concurrent execution with uart_change_line_settings(). In uart_throttle() and uart_unthrottle(), the check if (port->status & mask) is made, followed by mask &= ~port->status, where the relevant bits are cleared. However, port->status may be modified in uart_change_line_settings(). The current implementation does not ensure atomicity in the access and modification of port->status and mask. This can result in mask being updated based on a modified port->status value, leading to improper UART actions.
This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 5.17.
To resolve this double fetch, it is suggested to add a uart_port_lock pair in uart_throttle() and uart_unthrottle(). With this patch applied, our tool no longer reports the bug, with the kernel configuration allyesconfig for x86_64. Due to the absence of the requisite hardware, we are unable to conduct runtime testing of the patch. Therefore, our verification is solely based on code logic analysis.
[1] https://sites.google.com/view/basscheck/
Fixes: 391f93f2ec9f ("serial: core: Rework hw-assisted flow control support") Cc: stable@vger.kernel.org Signed-off-by: Gui-Dong Han 2045gemini@gmail.com
drivers/tty/serial/serial_core.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 80085b151b34..9d905fdf2843 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -723,11 +723,13 @@ static void uart_throttle(struct tty_struct *tty) mask |= UPSTAT_AUTOXOFF; if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS;
- uart_port_lock_irq(port); if (port->status & mask) { port->ops->throttle(port); mask &= ~port->status; }
- uart_port_unlock_irq(port);
You would also need to remove uart_port_lock_irq() out of all the throttle() callbacks.
if (mask & UPSTAT_AUTORTS) uart_clear_mctrl(port, TIOCM_RTS); @@ -753,10 +755,12 @@ static void uart_unthrottle(struct tty_struct *tty) if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS;
- uart_port_lock_irq(port); if (port->status & mask) { port->ops->unthrottle(port); mask &= ~port->status; }
- uart_port_unlock_irq(port);
You would also need to remove uart_port_lock_irq() out of all the unthrottle() callbacks.
John Ogness
On Fri, Jan 12, 2024 at 08:18:44PM +0800, Gui-Dong Han wrote:
In uart_throttle() and uart_unthrottle(): if (port->status & mask) { port->ops->throttle/unthrottle(port); mask &= ~port->status; } // Code segment utilizing the mask value to determine UART behavior
In uart_change_line_settings(): uart_port_lock_irq(uport); // Code segment responsible for updating uport->status uart_port_unlock_irq(uport);
In the uart_throttle() and uart_unthrottle() functions, there is a double fetch issue due to concurrent execution with uart_change_line_settings(). In uart_throttle() and uart_unthrottle(), the check if (port->status & mask) is made, followed by mask &= ~port->status, where the relevant bits are cleared. However, port->status may be modified in uart_change_line_settings(). The current implementation does not ensure atomicity in the access and modification of port->status and mask. This can result in mask being updated based on a modified port->status value, leading to improper UART actions.
What would be modifying the status and mask at the same point in time? Are you sure that it is possible do this?
This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 5.17.
5.17 is VERY old and obsolete, please work against 6.7 at the oldest. No one can take a patch for 5.17 anymore, you know this :(
thanks,
greg k-h
On Fri, 12 Jan 2024, Gui-Dong Han wrote:
In uart_throttle() and uart_unthrottle(): if (port->status & mask) { port->ops->throttle/unthrottle(port); mask &= ~port->status; } // Code segment utilizing the mask value to determine UART behavior
In uart_change_line_settings(): uart_port_lock_irq(uport); // Code segment responsible for updating uport->status uart_port_unlock_irq(uport);
In the uart_throttle() and uart_unthrottle() functions, there is a double fetch issue due to concurrent execution with uart_change_line_settings(). In uart_throttle() and uart_unthrottle(), the check if (port->status & mask) is made, followed by mask &= ~port->status, where the relevant bits are cleared. However, port->status may be modified in uart_change_line_settings(). The current implementation does not ensure atomicity in the access and modification of port->status and mask. This can result in mask being updated based on a modified port->status value, leading to improper UART actions.
This possible bug is found by an experimental static analysis tool developed by our team, BassCheck[1]. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations. The above possible bug is reported when our tool analyzes the source code of Linux 5.17.
To resolve this double fetch, it is suggested to add a uart_port_lock pair in uart_throttle() and uart_unthrottle(). With this patch applied, our tool no longer reports the bug, with the kernel configuration allyesconfig for x86_64. Due to the absence of the requisite hardware, we are unable to conduct runtime testing of the patch. Therefore, our verification is solely based on code logic analysis.
[1] https://sites.google.com/view/basscheck/
Fixes: 391f93f2ec9f ("serial: core: Rework hw-assisted flow control support") Cc: stable@vger.kernel.org Signed-off-by: Gui-Dong Han 2045gemini@gmail.com
drivers/tty/serial/serial_core.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 80085b151b34..9d905fdf2843 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -723,11 +723,13 @@ static void uart_throttle(struct tty_struct *tty) mask |= UPSTAT_AUTOXOFF; if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS;
- uart_port_lock_irq(port); if (port->status & mask) { port->ops->throttle(port); mask &= ~port->status; }
- uart_port_unlock_irq(port);
if (mask & UPSTAT_AUTORTS) uart_clear_mctrl(port, TIOCM_RTS); @@ -753,10 +755,12 @@ static void uart_unthrottle(struct tty_struct *tty) if (C_CRTSCTS(tty)) mask |= UPSTAT_AUTORTS;
- uart_port_lock_irq(port); if (port->status & mask) { port->ops->unthrottle(port); mask &= ~port->status; }
- uart_port_unlock_irq(port);
if (mask & UPSTAT_AUTORTS) uart_set_mctrl(port, TIOCM_RTS);
Hi,
This is very bogus "fix". While change to the local variable gets "protected", uart_change_line_settings() can race after unlock and the value held in mask is again stale.
If, and it's a big if, this is a real problem, the patch does not fix anything! It proves your tool is flawed because it doesn't detect the race with uart_change_line_settings() issue still exists after this non-fix.
So NAK from me. Please provide a real fix instead if you think there is a real issue.
Also, don't use vague wording like "leading to improper UART action" but describe precisely what goes wrong!
Hi
I apologize for any issues and lack of clarity in my previous patch. In patch v2, I've revised the fix to use local variables instead of locks and improved the description to clearly explain the harm and potential for concurrency.
The patch was developed and tested on linux-next, not Linux 5.17. My reference to 5.17 was due to a project I'm working on, which involves kernel static analysis and a comparison with earlier studies that support up to Linux 5.17. Therefore, I initially ran my tool on 5.17 to filter potential bugs that are still unaddressed in the upstream. Then I worked on linux-next to develop and test the patch. I understand this might have caused misunderstandings, and I'll aim for clearer communication in future submissions.
Thanks, Han
linux-stable-mirror@lists.linaro.org
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Greg KH -
Gui-Dong Han -
Ilpo Järvinen -
John Ogness