vt: Replace strlcpy with strscpy

PATCH In review Azeem Shaikh 3 participants 3 messages Sep 19, 2023 – Sep 20, 2023 TTY layer and serial drivers

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Azeem Shaikh

author

2023-09-19 19:22 UTC #0 lore raw

PATCH v2 1/1 vt: Replace strlcpy with strscpy

strlcpy() reads the entire source buffer first and returns the size of the source string, not the destination string, which can be accidentally misused [1].

The copy_to_user() call uses @len returned from strlcpy() directly without checking its value. This could potentially lead to read overflow. There is no existing bug since @len is always guaranteed to be greater than hardcoded strings in @func_table[kb_func]. But as written it is very fragile and specifically uses a strlcpy() result without sanity checking and using it to copy to userspace.

In an effort to remove strlcpy() completely [2], replace strlcpy() here with strscpy().

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89

Signed-off-by Azeem Shaikh <[email protected]>

---
v2:
* Return -ENOSPC instead of -EFAULT in case of truncation.
* Update commit log to clarify that there is no exploitable bug but instead the code uses a fragile anti-pattern.

v1:
* https://lore.kernel.org/all/[email protected]/

 drivers/tty/vt/keyboard.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

drivers/tty/vt/keyboard.c +5 -2

index 1fe6107b539b..12a192e1196b 100644
@@ -2079,12 +2079,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 			return -ENOMEM;
 
 		spin_lock_irqsave(&func_buf_lock, flags);
-		len = strlcpy(kbs, func_table[kb_func] ? : "", len);
+		len = strscpy(kbs, func_table[kb_func] ? : "", len);
 		spin_unlock_irqrestore(&func_buf_lock, flags);
 
+		if (len < 0) {
+			ret = -ENOSPC;
+			break;
+		}
 		ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
 			-EFAULT : 0;
-
 		break;
 	}
 	case KDSKBSENT:

--
2.42.0.459.ge4e396fd5e-goog

Justin Stitt

Reviewed-by on v2 1/1

2023-09-20 03:21 UTC #1 lore raw

Quoted 12 lines · Azeem Shaikh

strlcpy() reads the entire source buffer first and returns the size of the source string, not the destination string, which can be accidentally misused [1].
The copy_to_user() call uses @len returned from strlcpy() directly without checking its value. This could potentially lead to read overflow. There is no existing bug since @len is always guaranteed to be greater than hardcoded strings in @func_table[kb_func]. But as written it is very fragile and specifically uses a strlcpy() result without sanity checking and using it to copy to userspace.

Quoted 7 lines

In an effort to remove strlcpy() completely [2], replace strlcpy() here with strscpy().
[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89
Signed-off-by Azeem Shaikh <[email protected]>

Reviewed-by Justin Stitt <[email protected]>

Quoted 34 lines

---
v2:
* Return -ENOSPC instead of -EFAULT in case of truncation.
* Update commit log to clarify that there is no exploitable bug but instead the code uses a fragile anti-pattern.

v1:
* https://lore.kernel.org/all/[email protected]/

 drivers/tty/vt/keyboard.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

drivers/tty/vt/keyboard.c +5 -3

index 1fe6107b539b..12a192e1196b 100644
@@ -2079,12 +2079,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
                        return -ENOMEM;
 
                spin_lock_irqsave(&func_buf_lock, flags);
-               len = strlcpy(kbs, func_table[kb_func] ? : "", len);
+               len = strscpy(kbs, func_table[kb_func] ? : "", len);
                spin_unlock_irqrestore(&func_buf_lock, flags);
 
+               if (len < 0) {
+                       ret = -ENOSPC;
+                       break;
+               }
                ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
                        -EFAULT : 0;
-
                break;
        }
        case KDSKBSENT:
--

Quoted 3 lines

2.42.0.459.ge4e396fd5e-goog

whitespace nitpicks: A newline after the newly added if-statement's ending curly brace as well as reinstating the removed newline in your diff might make the code look "better".

Thanks
Justin

Kees Cook

Reviewed-by on v2 1/1

2023-09-20 15:13 UTC #2 lore raw

In reply to Azeem Shaikh

Quoted 14 lines · Azeem Shaikh

strlcpy() reads the entire source buffer first and returns the size of the source string, not the destination string, which can be accidentally misused [1].
The copy_to_user() call uses @len returned from strlcpy() directly without checking its value. This could potentially lead to read overflow. There is no existing bug since @len is always guaranteed to be greater than hardcoded strings in @func_table[kb_func]. But as written it is very fragile and specifically uses a strlcpy() result without sanity checking and using it to copy to userspace.
In an effort to remove strlcpy() completely [2], replace strlcpy() here with strscpy().

Quoted 8 lines

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89
Signed-off-by Azeem Shaikh <[email protected]>
---
v2:
* Return -ENOSPC instead of -EFAULT in case of truncation.
* Update commit log to clarify that there is no exploitable bug but instead the code uses a fragile anti-pattern.

Changes look good. Thanks!

Reviewed-by Kees Cook <[email protected]>

Signature · 1 line

Kees Cook