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(-)
diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 1fe6107b539b..12a192e1196b 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -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
On Tue, Sep 19, 2023 at 12:22 PM Azeem Shaikh <[email protected]> wrote:
>
> 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]>
Reviewed-by: Justin Stitt <[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(-)
>
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index 1fe6107b539b..12a192e1196b 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -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
>
>
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
On Tue, Sep 19, 2023 at 07:21:56PM +0000, Azeem Shaikh wrote:
> 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.
Changes look good. Thanks!
Reviewed-by: Kees Cook <[email protected]>
--
Kees Cook