Discussion:
[PATCH] target-ppc: kvm: Fix memory overflow issue about strncat()
Chen Gang
2014-10-13 14:36:02 UTC
Permalink
strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.

Signed-off-by: Chen Gang <***@gmail.com>
---
target-ppc/kvm.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..66e7ce5 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,8 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}

- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ strncat(buf, "/", sizeof(buf) - strlen(buf) - 1);
+ strncat(buf, propname, sizeof(buf) - strlen(buf) - 1);

f = fopen(buf, "rb");
if (!f) {
--
1.9.3
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Alexander Graf
2014-10-13 14:47:50 UTC
Permalink
Post by Chen Gang
strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.
I agree with this patch. However, the code is pretty ugly - I'm sure it
must've been me who wrote it :).

Could you please instead rewrite it to use g_strdup_printf() rather than
strncat()s? That way we resolve all string pitfalls automatically - and
this code is not the fast path, so doing an extra memory allocation is ok.


Alex
Post by Chen Gang
---
target-ppc/kvm.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..66e7ce5 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,8 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}
- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ strncat(buf, "/", sizeof(buf) - strlen(buf) - 1);
+ strncat(buf, propname, sizeof(buf) - strlen(buf) - 1);
f = fopen(buf, "rb");
if (!f) {
--
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Chen Gang
2014-10-13 15:43:43 UTC
Permalink
Post by Alexander Graf
Could you please instead rewrite it to use g_strdup_printf() rather than
strncat()s? That way we resolve all string pitfalls automatically - and
this code is not the fast path, so doing an extra memory allocation is ok.
I guess, it is a personal taste. For me, it may need additional variable
for g_strdup_printf(), and not save code lines, but *sprintf() is more
readable than str*cat().

The related code may like below (welcome any improvement for it):

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 66e7ce5..cea6a87 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1782,7 +1782,7 @@ static int kvmppc_find_cpu_dt(char *buf, int buf_len)
* format) */
static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
{
- char buf[PATH_MAX];
+ char buf[PATH_MAX], *tmp;
union {
uint32_t v32;
uint64_t v64;
@@ -1794,10 +1794,10 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}

- strncat(buf, "/", sizeof(buf) - strlen(buf) - 1);
- strncat(buf, propname, sizeof(buf) - strlen(buf) - 1);
+ tmp = g_strdup_printf("%s/%s", buf, propname);

- f = fopen(buf, "rb");
+ f = fopen(tmp, "rb");
+ g_free(buf);
if (!f) {
return -1;
}

For me, it is really a personal taste, so if the maintainer feels the
diff above is OK, I shall send patch v2 for it within 2 days.


Thanks.
--
Chen Gang

Open, share, and attitude like air, water, and life which God blessed
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Michael Tokarev
2014-10-24 07:49:01 UTC
Permalink
Post by Alexander Graf
Post by Chen Gang
strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.
I agree with this patch. However, the code is pretty ugly - I'm sure it
must've been me who wrote it :).
Could you please instead rewrite it to use g_strdup_printf() rather than
strncat()s? That way we resolve all string pitfalls automatically - and
this code is not the fast path, so doing an extra memory allocation is ok.
I'd just use snprintf() like this:

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..5eaa36c 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,7 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}

- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "/%s", propname);

f = fopen(buf, "rb");
if (!f) {

the buffer is of size PATH_MAX, and we're looking at /proc filesystem where
names should be rather short so we're extremly unlikely to hit this prob in
practice, there's no need to dynamically allocate a buffer for this stuff.

(Or alternatively there's asprintf(), but still I think it is overkill).

I can apply the above if everyone agrees.

Thanks,

/mjt
Post by Alexander Graf
Post by Chen Gang
target-ppc/kvm.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..66e7ce5 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,8 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}
- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ strncat(buf, "/", sizeof(buf) - strlen(buf) - 1);
+ strncat(buf, propname, sizeof(buf) - strlen(buf) - 1);
f = fopen(buf, "rb");
if (!f) {
--
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Alexander Graf
2014-10-24 08:05:12 UTC
Permalink
Post by Chen Gang
Post by Alexander Graf
Post by Chen Gang
strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.
I agree with this patch. However, the code is pretty ugly - I'm sure it
must've been me who wrote it :).
Could you please instead rewrite it to use g_strdup_printf() rather than
strncat()s? That way we resolve all string pitfalls automatically - and
this code is not the fast path, so doing an extra memory allocation is ok.
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..5eaa36c 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,7 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}
- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "/%s", propname);
f = fopen(buf, "rb");
if (!f) {
the buffer is of size PATH_MAX, and we're looking at /proc filesystem where
names should be rather short so we're extremly unlikely to hit this prob in
practice, there's no need to dynamically allocate a buffer for this stuff.
I've already applied the fix to ppc-next.
Post by Chen Gang
(Or alternatively there's asprintf(), but still I think it is overkill).
That one isn't portable iirc.
Post by Chen Gang
I can apply the above if everyone agrees.
No worries, it's all over already ;).

Alex

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Chen Gang
2014-10-24 08:19:43 UTC
Permalink
Post by Chen Gang
Post by Alexander Graf
Post by Chen Gang
strncat() will append additional '\0' to destination buffer, so need
additional 1 byte for it, or may cause memory overflow, just like other
area within QEMU have done.
I agree with this patch. However, the code is pretty ugly - I'm sure it
must've been me who wrote it :).
Could you please instead rewrite it to use g_strdup_printf() rather than
strncat()s? That way we resolve all string pitfalls automatically - and
this code is not the fast path, so doing an extra memory allocation is ok.
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 9c23c6b..5eaa36c 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -1794,8 +1794,7 @@ static uint64_t kvmppc_read_int_cpu_dt(const char *propname)
return -1;
}
- strncat(buf, "/", sizeof(buf) - strlen(buf));
- strncat(buf, propname, sizeof(buf) - strlen(buf));
+ snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf), "/%s", propname);
f = fopen(buf, "rb");
if (!f) {
the buffer is of size PATH_MAX, and we're looking at /proc filesystem where
names should be rather short so we're extremly unlikely to hit this prob in
practice, there's no need to dynamically allocate a buffer for this stuff.
(Or alternatively there's asprintf(), but still I think it is overkill).
I can apply the above if everyone agrees.
For me, what you said is reasonable, although I am not sure whether the
patch v2 for g_strdup_printf() was already applied or not.

Thanks.
--
Chen Gang

Open, share, and attitude like air, water, and life which God blessed
--
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