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Date: Thu, 14 May 2020 18:16:07 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: ast@...nel.org
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
torvalds@...ux-foundation.org, mhiramat@...nel.org,
brendan.d.gregg@...il.com, hch@....de, john.fastabend@...il.com,
yhs@...com, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH bpf 3/3] bpf: restrict bpf_trace_printk()'s %s usage and add %psK, %psU specifier
Usage of plain %s conversion specifier in bpf_trace_printk() suffers from the
very same issue as bpf_probe_read{,str}() helpers, that is, it is broken on
archs with overlapping address ranges.
While the helpers have been addressed through work in 6ae08ae3dea2 ("bpf: Add
probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers"), we need
an option for bpf_trace_printk() as well to fix it.
Similarly as with the helpers, force users to make an explicit choice by adding
%psK and %psU specifier to bpf_trace_printk() which will then pick the corresponding
strncpy_from_unsafe*() variant to perform the access under KERNEL_DS or USER_DS.
Existing %s for legacy users is still kept working for archs where it is not
broken and therefore gated through CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
Fixes: 8d3b7dce8622 ("bpf: add support for %s specifier to bpf_trace_printk()")
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Reported-by: Christoph Hellwig <hch@....de>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Brendan Gregg <brendan.d.gregg@...il.com>
Cc: Christoph Hellwig <hch@....de>
---
Documentation/core-api/printk-formats.rst | 14 ++++
kernel/trace/bpf_trace.c | 92 +++++++++++++++--------
lib/vsprintf.c | 7 +-
3 files changed, 81 insertions(+), 32 deletions(-)
diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 8ebe46b1af39..76b5f4f265cb 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -112,6 +112,20 @@ used when printing stack backtraces. The specifier takes into
consideration the effect of compiler optimisations which may occur
when tail-calls are used and marked with the noreturn GCC attribute.
+Probed Strings from BPF
+-----------------------
+
+::
+
+ %psK kernel_string
+ %psU user_string
+
+The ``sK`` and ``sU`` specifiers are used for printing a string from probed
+memory. From regular vsnprintf(), they are equivalent to ``%s``, however,
+when used out of BPF's bpf_trace_printk() it reads a string of up to 64 bytes
+in memory without faulting. For ``K`` specifier, the string is probed out of
+kernel memory whereas for ``U`` specifier, it is probed out of user memory.
+
Kernel Pointers
---------------
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b83bdaa31c7b..9eef2075ea18 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -323,17 +323,15 @@ static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
/*
* Only limited trace_printk() conversion specifiers allowed:
- * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %s
+ * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %psK %psU %s
*/
BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
u64, arg2, u64, arg3)
{
+ int i, mod[3] = {}, fmt_cnt = 0;
+ void *unsafe_ptr = NULL;
bool str_seen = false;
- int mod[3] = {};
- int fmt_cnt = 0;
- u64 unsafe_addr;
char buf[64];
- int i;
/*
* bpf_check()->check_func_arg()->check_stack_boundary()
@@ -359,40 +357,71 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
if (fmt[i] == 'l') {
mod[fmt_cnt]++;
i++;
- } else if (fmt[i] == 'p' || fmt[i] == 's') {
+ } else if (fmt[i] == 'p') {
mod[fmt_cnt]++;
+ if (fmt[i + 1] == 's' &&
+ (fmt[i + 2] == 'K' ||
+ fmt[i + 2] == 'U')) {
+ i += 2;
+ goto fmt_str;
+ }
+
/* disallow any further format extensions */
if (fmt[i + 1] != 0 &&
!isspace(fmt[i + 1]) &&
!ispunct(fmt[i + 1]))
return -EINVAL;
- fmt_cnt++;
- if (fmt[i] == 's') {
- if (str_seen)
- /* allow only one '%s' per fmt string */
- return -EINVAL;
- str_seen = true;
-
- switch (fmt_cnt) {
- case 1:
- unsafe_addr = arg1;
- arg1 = (long) buf;
- break;
- case 2:
- unsafe_addr = arg2;
- arg2 = (long) buf;
- break;
- case 3:
- unsafe_addr = arg3;
- arg3 = (long) buf;
- break;
- }
- buf[0] = 0;
- strncpy_from_unsafe(buf,
- (void *) (long) unsafe_addr,
+
+ goto fmt_next;
+ } else if (fmt[i] == 's') {
+ mod[fmt_cnt]++;
+fmt_str:
+ if (str_seen)
+ /* allow only one '%s' per fmt string */
+ return -EINVAL;
+ str_seen = true;
+
+ if (fmt[i + 1] != 0 &&
+ !isspace(fmt[i + 1]) &&
+ !ispunct(fmt[i + 1]))
+ return -EINVAL;
+
+ switch (fmt_cnt) {
+ case 1:
+ unsafe_ptr = (void *)(long)arg1;
+ arg1 = (long)buf;
+ break;
+ case 2:
+ unsafe_ptr = (void *)(long)arg2;
+ arg2 = (long)buf;
+ break;
+ case 3:
+ unsafe_ptr = (void *)(long)arg3;
+ arg3 = (long)buf;
+ break;
+ }
+
+ buf[0] = 0;
+
+ switch (fmt[i]) {
+ default:
+ return -EOPNOTSUPP;
+#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
+ case 's':
+ /* Fallthrough */
+#endif
+ case 'K':
+ strncpy_from_unsafe(buf, unsafe_ptr,
sizeof(buf));
+ break;
+ case 'U':
+ strncpy_from_unsafe_user(buf,
+ (__force void __user *)unsafe_ptr,
+ sizeof(buf));
+ break;
}
- continue;
+
+ goto fmt_next;
}
if (fmt[i] == 'l') {
@@ -403,6 +432,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
if (fmt[i] != 'i' && fmt[i] != 'd' &&
fmt[i] != 'u' && fmt[i] != 'x')
return -EINVAL;
+fmt_next:
fmt_cnt++;
}
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a1ce318..06161925225b 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2168,6 +2168,8 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
* f full name
* P node name, including a possible unit address
* - 'x' For printing the address. Equivalent to "%lx".
+ * - 's[KU]' For printing a string, used in bpf_trace_printk(). For non-BPF
+ * context this is equivalent to "%s".
*
* ** When making changes please also update:
* Documentation/core-api/printk-formats.rst
@@ -2180,8 +2182,11 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
struct printf_spec spec)
{
switch (*fmt) {
- case 'S':
case 's':
+ if (fmt[1] == 'K' || fmt[1] == 'U')
+ return string(buf, end, ptr, spec);
+ /* Fallthrough */
+ case 'S':
ptr = dereference_symbol_descriptor(ptr);
/* Fallthrough */
case 'B':
--
2.21.0
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