kernel/trace/trace_events_filter.c | 10 ++++++++-- mm/maccess.c | 2 +- 2 files changed, 9 insertions(+), 3 deletions(-)
From: Mykyta Yatsenko <yatsenko@meta.com>
strncpy_from_user_nofault should return the length of the copied string
including the trailing NUL, but if the argument unsafe_addr points to
an empty string ({'\0'}), the return value is 0.
This happens as strncpy_from_user copies terminal symbol into dst
and returns 0 (as expected), but strncpy_from_user_nofault does not
modify ret as it is not equal to count and not greater than 0, so 0 is
returned, which contradicts the contract.
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
kernel/trace/trace_events_filter.c | 10 ++++++++--
mm/maccess.c | 2 +-
2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 0993dfc1c5c1..86b7e5a4e235 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -800,6 +800,7 @@ static __always_inline char *test_string(char *str)
{
struct ustring_buffer *ubuf;
char *kstr;
+ int cnt;
if (!ustring_per_cpu)
return NULL;
@@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
kstr = ubuf->buffer;
/* For safety, do not trust the string pointer */
- if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
+ cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
+ /* Return null if empty string or error */
+ if (cnt <= 1)
return NULL;
return kstr;
}
@@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
struct ustring_buffer *ubuf;
char __user *ustr;
char *kstr;
+ int cnt;
if (!ustring_per_cpu)
return NULL;
@@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
/* user space address? */
ustr = (char __user *)str;
- if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
+ cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
+ /* Return null if empty string or error */
+ if (cnt <= 1)
return NULL;
return kstr;
diff --git a/mm/maccess.c b/mm/maccess.c
index 8f0906180a94..831b4dd7296c 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
if (ret >= count) {
ret = count;
dst[ret - 1] = '\0';
- } else if (ret > 0) {
+ } else if (ret >= 0) {
ret++;
}
--
2.49.0On Thu, Apr 17, 2025 at 8:28 AM Mykyta Yatsenko
<mykyta.yatsenko5@gmail.com> wrote:
>
> From: Mykyta Yatsenko <yatsenko@meta.com>
>
> strncpy_from_user_nofault should return the length of the copied string
> including the trailing NUL, but if the argument unsafe_addr points to
> an empty string ({'\0'}), the return value is 0.
>
> This happens as strncpy_from_user copies terminal symbol into dst
> and returns 0 (as expected), but strncpy_from_user_nofault does not
> modify ret as it is not equal to count and not greater than 0, so 0 is
> returned, which contradicts the contract.
>
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---
> kernel/trace/trace_events_filter.c | 10 ++++++++--
> mm/maccess.c | 2 +-
> 2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 0993dfc1c5c1..86b7e5a4e235 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -800,6 +800,7 @@ static __always_inline char *test_string(char *str)
> {
> struct ustring_buffer *ubuf;
> char *kstr;
> + int cnt;
>
> if (!ustring_per_cpu)
> return NULL;
> @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
> kstr = ubuf->buffer;
>
> /* For safety, do not trust the string pointer */
> - if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
> + cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
> + /* Return null if empty string or error */
> + if (cnt <= 1)
> return NULL;
I wouldn't touch this part and leave it up to Steven to fix (if he
agrees it needs fixing). Current logic seems wrong already, as it
won't correctly handle -EFAULT. And, on the other hand, there is
nothing wrong or special about empty string, so I don't think it needs
special handling. Let's drop these changes in trace_events_filter.c?
> return kstr;
> }
> @@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
> struct ustring_buffer *ubuf;
> char __user *ustr;
> char *kstr;
> + int cnt;
>
> if (!ustring_per_cpu)
> return NULL;
> @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
>
> /* user space address? */
> ustr = (char __user *)str;
> - if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
> + cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
> + /* Return null if empty string or error */
> + if (cnt <= 1)
> return NULL;
ditto
>
> return kstr;
> diff --git a/mm/maccess.c b/mm/maccess.c
> index 8f0906180a94..831b4dd7296c 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
> if (ret >= count) {
> ret = count;
> dst[ret - 1] = '\0';
> - } else if (ret > 0) {
> + } else if (ret >= 0) {
> ret++;
> }
>
This part looks good and does indeed fix the issue. Good catch!
Reviewed-by: Andrii Nakryiko <andrii@kernel.org>
> --
> 2.49.0
>
On Thu, 17 Apr 2025 13:44:48 -0700 Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote: > > @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str) > > kstr = ubuf->buffer; > > > > /* For safety, do not trust the string pointer */ > > - if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE)) > > + cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE); > > + /* Return null if empty string or error */ > > + if (cnt <= 1) > > return NULL; > > I wouldn't touch this part and leave it up to Steven to fix (if he > agrees it needs fixing). Current logic seems wrong already, as it > won't correctly handle -EFAULT. And, on the other hand, there is > nothing wrong or special about empty string, so I don't think it needs > special handling. Let's drop these changes in trace_events_filter.c? Bah, it is wrong. I don't usually use filtering on strings much, but come to think of it, the last time I tried, it didn't work, but I found another way to get what I was looking for, and didn't look deeper into it. I only care if it faulted or not. I don't care about it just copying zero bytes. It should have been: if (strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE) < 0) > > > return kstr; > > } > > @@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str) > > struct ustring_buffer *ubuf; > > char __user *ustr; > > char *kstr; > > + int cnt; > > > > if (!ustring_per_cpu) > > return NULL; > > @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str) > > > > /* user space address? */ > > ustr = (char __user *)str; > > - if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE)) This is broken too. As this isn't relying on the other change in this patch, I'll just fix it myself. I'm getting a pull request ready anyway. Thanks! -- Steve > > + cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE); > > + /* Return null if empty string or error */ > > + if (cnt <= 1) > > return NULL; > > ditto > > >
On Thu, Apr 17, 2025 at 1:44 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Apr 17, 2025 at 8:28 AM Mykyta Yatsenko
> <mykyta.yatsenko5@gmail.com> wrote:
> >
> > From: Mykyta Yatsenko <yatsenko@meta.com>
> >
> > strncpy_from_user_nofault should return the length of the copied string
> > including the trailing NUL, but if the argument unsafe_addr points to
> > an empty string ({'\0'}), the return value is 0.
> >
> > This happens as strncpy_from_user copies terminal symbol into dst
> > and returns 0 (as expected), but strncpy_from_user_nofault does not
> > modify ret as it is not equal to count and not greater than 0, so 0 is
> > returned, which contradicts the contract.
> >
> > Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> > ---
> > kernel/trace/trace_events_filter.c | 10 ++++++++--
> > mm/maccess.c | 2 +-
> > 2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> > index 0993dfc1c5c1..86b7e5a4e235 100644
> > --- a/kernel/trace/trace_events_filter.c
> > +++ b/kernel/trace/trace_events_filter.c
> > @@ -800,6 +800,7 @@ static __always_inline char *test_string(char *str)
> > {
> > struct ustring_buffer *ubuf;
> > char *kstr;
> > + int cnt;
> >
> > if (!ustring_per_cpu)
> > return NULL;
> > @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
> > kstr = ubuf->buffer;
> >
> > /* For safety, do not trust the string pointer */
> > - if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
> > + cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
> > + /* Return null if empty string or error */
> > + if (cnt <= 1)
> > return NULL;
>
> I wouldn't touch this part and leave it up to Steven to fix (if he
> agrees it needs fixing). Current logic seems wrong already, as it
> won't correctly handle -EFAULT. And, on the other hand, there is
> nothing wrong or special about empty string, so I don't think it needs
> special handling. Let's drop these changes in trace_events_filter.c?
Actually, you are not even touching strncpy_from_kernel_nofault(), so
yeah, definitely let's not do this (at least not in this patch).
>
> > return kstr;
> > }
> > @@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
> > struct ustring_buffer *ubuf;
> > char __user *ustr;
> > char *kstr;
> > + int cnt;
> >
> > if (!ustring_per_cpu)
> > return NULL;
> > @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
> >
> > /* user space address? */
> > ustr = (char __user *)str;
> > - if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
> > + cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
> > + /* Return null if empty string or error */
> > + if (cnt <= 1)
> > return NULL;
>
> ditto
>
> >
> > return kstr;
> > diff --git a/mm/maccess.c b/mm/maccess.c
> > index 8f0906180a94..831b4dd7296c 100644
> > --- a/mm/maccess.c
> > +++ b/mm/maccess.c
> > @@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
> > if (ret >= count) {
> > ret = count;
> > dst[ret - 1] = '\0';
> > - } else if (ret > 0) {
> > + } else if (ret >= 0) {
> > ret++;
> > }
> >
>
> This part looks good and does indeed fix the issue. Good catch!
>
> Reviewed-by: Andrii Nakryiko <andrii@kernel.org>
>
> > --
> > 2.49.0
> >
On Thu, 17 Apr 2025 16:28:08 +0100 Mykyta Yatsenko <mykyta.yatsenko5@gmail.com> wrote:
> strncpy_from_user_nofault should return the length of the copied string
> including the trailing NUL, but if the argument unsafe_addr points to
> an empty string ({'\0'}), the return value is 0.
>
> This happens as strncpy_from_user copies terminal symbol into dst
> and returns 0 (as expected), but strncpy_from_user_nofault does not
> modify ret as it is not equal to count and not greater than 0, so 0 is
> returned, which contradicts the contract.
Looks right, I think.
But why do strncpy_from_user() and strncpy_from_user_nofault() have
different interfaces?
/**
* strncpy_from_user: - Copy a NUL terminated string from userspace.
* ...
* On success, returns the length of the string (not including the trailing
* NUL).
/**
* strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
* address.
* ...
* On success, returns the length of the string INCLUDING the trailing NUL.
This is surprising and I'm wondering what led us to do this?
On Thu, Apr 17, 2025 at 1:40 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 17 Apr 2025 16:28:08 +0100 Mykyta Yatsenko <mykyta.yatsenko5@gmail.com> wrote:
>
> > strncpy_from_user_nofault should return the length of the copied string
> > including the trailing NUL, but if the argument unsafe_addr points to
> > an empty string ({'\0'}), the return value is 0.
> >
> > This happens as strncpy_from_user copies terminal symbol into dst
> > and returns 0 (as expected), but strncpy_from_user_nofault does not
> > modify ret as it is not equal to count and not greater than 0, so 0 is
> > returned, which contradicts the contract.
>
> Looks right, I think.
>
> But why do strncpy_from_user() and strncpy_from_user_nofault() have
> different interfaces?
>
> /**
> * strncpy_from_user: - Copy a NUL terminated string from userspace.
> * ...
> * On success, returns the length of the string (not including the trailing
> * NUL).
>
> /**
> * strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
> * address.
> * ...
> * On success, returns the length of the string INCLUDING the trailing NUL.
>
> This is surprising and I'm wondering what led us to do this?
Agreed, this is very surprising and error-prone. strncpy_from_user()
semantics is a bit better, IMO, in that it allows to "detect" empty
string even if buffer size is 1 byte. And there isn't a lot of places
where we use strncpy_from_user_nofault (only 6, it seems). Maybe we
should just change the semantics of strncpy_from_user_nofault?
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