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Thu, 17 Apr 2025 08:28:25 -0700 (PDT) Received: from msi-laptop.thefacebook.com ([2620:10d:c092:500::7:4ad]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-acb6ef475d7sm7154666b.135.2025.04.17.08.28.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Apr 2025 08:28:24 -0700 (PDT) From: Mykyta Yatsenko To: akpm@linux-foundation.org, linux-mm@kvack.org, rostedt@goodmis.org, mhiramat@kernel.org, andrii@kernel.org, kernel-team@meta.com, linux-kernel@vger.kernel.org Cc: Mykyta Yatsenko Subject: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling Date: Thu, 17 Apr 2025 16:28:08 +0100 Message-ID: <20250417152808.722409-1-mykyta.yatsenko5@gmail.com> X-Mailer: git-send-email 2.49.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Mykyta Yatsenko 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 Reviewed-by: Andrii Nakryiko --- 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; =20 if (!ustring_per_cpu) return NULL; @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str) kstr =3D ubuf->buffer; =20 /* For safety, do not trust the string pointer */ - if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE)) + cnt =3D strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE); + /* Return null if empty string or error */ + if (cnt <=3D 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; =20 if (!ustring_per_cpu) return NULL; @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str) =20 /* user space address? */ ustr =3D (char __user *)str; - if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE)) + cnt =3D strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE); + /* Return null if empty string or error */ + if (cnt <=3D 1) return NULL; =20 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 >=3D count) { ret =3D count; dst[ret - 1] =3D '\0'; - } else if (ret > 0) { + } else if (ret >=3D 0) { ret++; } =20 --=20 2.49.0