| Message ID | 20230919192156.121503-1-azeems@google.com (mailing list archive) |
|---|---|
| State | Mainlined |
| Commit | 95e8e7eebab58ec8f6c978242a6d143ba238a69e |
| Headers | show |
| Series | [v2] vt: Replace strlcpy with strscpy | expand |
On Tue, Sep 19, 2023 at 12:22 PM Azeem Shaikh <azeems@google.com> 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 <azeems@google.com> Reviewed-by: Justin Stitt <justinstitt@google.com> > --- > 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/20230830160410.3820390-1-azeemshaikh38@gmail.com/ > > 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 <azeems@google.com> > --- > 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 <keescook@chromium.org>
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:
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 <azeems@google.com> --- 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/20230830160410.3820390-1-azeemshaikh38@gmail.com/ drivers/tty/vt/keyboard.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) -- 2.42.0.459.ge4e396fd5e-goog