gpiolib: Fix potential Spectre v1 vulnerabilities

offset and lineinfo.line_offset are indirectly controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/gpio/gpiolib.c:580 linehandle_create() warn: potential spectre issue 'gdev->descs' [r] (local cap) drivers/gpio/gpiolib.c:927 lineevent_create() warn: potential spectre issue 'gdev->descs' [r] (local cap) drivers/gpio/gpiolib.c:1053 gpio_ioctl() warn: potential spectre issue 'gdev->descs' [r] (local cap)

Fix this by sanitizing both offset and lineinfo.line_offset before using them to index gdev->descs.

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 drivers/gpio/gpiolib.c | 6 ++++++
 1 file changed, 6 insertions(+)

index fc36dc358716..73a0a550162e 100644
@@ -29,6 +29,8 @@
 #include <linux/timekeeping.h>
 #include <uapi/linux/gpio.h>
 
+#include <linux/nospec.h>
+
 #include "gpiolib.h"
 
 #define CREATE_TRACE_POINTS

@@ -576,6 +578,7 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
 			ret = -EINVAL;
 			goto out_free_descs;
 		}
+		offset = array_index_nospec(offset, gdev->ngpio);
 
 		desc = &gdev->descs[offset];
 		ret = gpiod_request(desc, lh->label);

@@ -910,6 +913,7 @@ static int lineevent_create(struct gpio_device *gdev, void __user *ip)
 		ret = -EINVAL;
 		goto out_free_label;
 	}
+	offset = array_index_nospec(offset, gdev->ngpio);
 
 	/* Return an error if a unknown flag is set */
 	if ((lflags & ~GPIOHANDLE_REQUEST_VALID_FLAGS) ||

@@ -1049,6 +1053,8 @@ static long gpio_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EFAULT;
 		if (lineinfo.line_offset >= gdev->ngpio)
 			return -EINVAL;
+		lineinfo.line_offset = array_index_nospec(lineinfo.line_offset,
+							  gdev->ngpio);
 
 		desc = &gdev->descs[lineinfo.line_offset];
 		if (desc->name) {

2.19.2

Goodness gracious me!

Bartosz can you take this for a ride with libgpiod and see if we get performance regressions from this speculation killing?

If you get some data on that I would like to include it with the changelog.

Yours,
Linus Walleij

I don't know what to do with this patch and I am also worried that it is dangerous and irresponsible to drop it.

I am worried because I simply don't understand the conditions under which this can be exploited, to what extent, and I am worried that I am sacrificing performance for security that is not needed on this performance-critical path, which is why I am asking for performance
effects: I really would like it if someone could take this for a ride on some x86 system with the mockdev and libgpiod.

It's not like gdev->descs is a secret or contain anything sensitive, the typical info is just electronic set-up. I get the idea from the commit message that this is all that may be exposed?

If this can be exploited to expose the whole of kernel memory on the other hand ("speculation windows are large"), I would be an idiot if I don't apply it. But then the commit message would say that the entire memory is at risk?

Yours,
Linus Walleij