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bpf: Restrict bpf_trace_printk()'s %s usage and add %pks, %pus 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 6ae08ae3de ("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
%pks and %pus 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.
The %pk* (kernel specifier) and %pu* (user specifier) can later also be extended
for other objects aside strings that are probed and printed under tracing, and
reused out of other facilities like bpf_seq_printf() or BTF based type printing.

Existing behavior of %s for current users is still kept working for archs where it
is not broken and therefore gated through CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
For archs not having this property we fall-back to pick probing under KERNEL_DS as
a sensible default.

Fixes: 8d3b7dce86 ("bpf: add support for %s specifier to bpf_trace_printk()")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Link: https://lore.kernel.org/bpf/20200515101118.6508-4-daniel@iogearbox.net
This commit is contained in:
Daniel Borkmann 2020-05-15 12:11:18 +02:00 committed by Alexei Starovoitov
parent 47cc0ed574
commit b2a5212fb6
3 changed files with 88 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
Documentation/core-api/printk-formats.rst
View File

@ -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 Pointers from BPF / tracing
----------------------------------
::
%pks kernel string
%pus user string
The ``k`` and ``u`` specifiers are used for printing prior probed memory from
either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
results in printing a string. For direct use in regular vsnprintf() the (k)
and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
for example, it reads the memory it is pointing to without faulting.
Kernel Pointers
---------------

94
kernel/trace/bpf_trace.c
View File

@ -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 %pks %pus %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;
char buf[64], fmt_ptype;
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] == 'k' ||
fmt[i + 1] == 'u') &&
fmt[i + 2] == 's') {
fmt_ptype = fmt[i + 1];
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,
sizeof(buf));
goto fmt_next;
} else if (fmt[i] == 's') {
mod[fmt_cnt]++;
fmt_ptype = fmt[i];
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 0:
unsafe_ptr = (void *)(long)arg1;
arg1 = (long)buf;
break;
case 1:
unsafe_ptr = (void *)(long)arg2;
arg2 = (long)buf;
break;
case 2:
unsafe_ptr = (void *)(long)arg3;
arg3 = (long)buf;
break;
}
continue;
buf[0] = 0;
switch (fmt_ptype) {
case 's':
#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
strncpy_from_unsafe(buf, unsafe_ptr,
sizeof(buf));
break;
#endif
case 'k':
strncpy_from_unsafe_strict(buf, unsafe_ptr,
sizeof(buf));
break;
case 'u':
strncpy_from_unsafe_user(buf,
(__force void __user *)unsafe_ptr,
sizeof(buf));
break;
}
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++;
}

12
lib/vsprintf.c
View File

@ -2168,6 +2168,10 @@ 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".
* - '[ku]s' For a BPF/tracing related format specifier, e.g. used out of
* bpf_trace_printk() where [ku] prefix specifies either kernel (k)
* or user (u) memory to probe, and:
* s a string, equivalent to "%s" on direct vsnprintf() use
*
* ** When making changes please also update:
* Documentation/core-api/printk-formats.rst
@ -2251,6 +2255,14 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
if (!IS_ERR(ptr))
break;
return err_ptr(buf, end, ptr, spec);
case 'u':
case 'k':
switch (fmt[1]) {
case 's':
return string(buf, end, ptr, spec);
default:
return error_string(buf, end, "(einval)", spec);
}
}
/* default is to _not_ leak addresses, hash before printing */