[PATCH 5.4 076/191] MIPS: BPF: Disable MIPS32 eBPF JIT

[PATCH 5.4 076/191] MIPS: BPF: Disable MIPS32 eBPF JIT

[Greg Kroah-Hartman]

From: Paul Burton <paulburton@kernel.org>

commit f8fffebdea752a25757b906f3dffecf1a59a6194 upstream.

Commit 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32
architecture.") enabled our eBPF JIT for MIPS32 kernels, whereas it has
previously only been availailable for MIPS64. It was my understanding at
the time that the BPF test suite was passing & JITing a comparable
number of tests to our cBPF JIT [1], but it turns out that was not the
case.

The eBPF JIT has a number of problems on MIPS32:

- Most notably various code paths still result in emission of MIPS64
  instructions which will cause reserved instruction exceptions & kernel
  panics when run on MIPS32 CPUs.

- The eBPF JIT doesn't account for differences between the O32 ABI used
  by MIPS32 kernels versus the N64 ABI used by MIPS64 kernels. Notably
  arguments beyond the first 4 are passed on the stack in O32, and this
  is entirely unhandled when JITing a BPF_CALL instruction. Stack space
  must be reserved for arguments even if they all fit in registers, and
  the callee is free to assume that stack space has been reserved for
  its use - with the eBPF JIT this is not the case, so calling any
  function can result in clobbering values on the stack & unpredictable
  behaviour. Function arguments in eBPF are always 64-bit values which
  is also entirely unhandled - the JIT still uses a single (32-bit)
  register per argument. As a result all function arguments are always
  passed incorrectly when JITing a BPF_CALL instruction, leading to
  kernel crashes or strange behavior.

- The JIT attempts to bail our on use of ALU64 instructions or 64-bit
  memory access instructions. The code doing this at the start of
  build_one_insn() incorrectly checks whether BPF_OP() equals BPF_DW,
  when it should really be checking BPF_SIZE() & only doing so when
  BPF_CLASS() is one of BPF_{LD,LDX,ST,STX}. This results in false
  positives that cause more bailouts than intended, and that in turns
  hides some of the problems described above.

- The kernel's cBPF->eBPF translation makes heavy use of 64-bit eBPF
  instructions that the MIPS32 eBPF JIT bails out on, leading to most
  cBPF programs not being JITed at all.

Until these problems are resolved, revert the enabling of the eBPF JIT
on MIPS32 done by commit 716850ab104d ("MIPS: eBPF: Initial eBPF support
for MIPS32 architecture.").

Note that this does not undo the changes made to the eBPF JIT by that
commit, since they are a useful starting point to providing MIPS32
support - they're just not nearly complete.

[1] https://lore.kernel.org/linux-mips/MWHPR2201MB13583388481F01A422CE7D66D4410@MWHPR2201MB1358.namprd22.prod.outlook.com/

Signed-off-by: Paul Burton <paulburton@kernel.org>
Fixes: 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32 architecture.")
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Hassan Naveed <hnaveed@wavecomp.com>
Cc: Tony Ambardar <itugrok@yahoo.com>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: <stable@vger.kernel.org> # v5.2+
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/mips/Kconfig |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -46,7 +46,7 @@ config MIPS
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES
 	select HAVE_ASM_MODVERSIONS
-	select HAVE_EBPF_JIT if (!CPU_MICROMIPS)
+	select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
 	select HAVE_CONTEXT_TRACKING
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_C_RECORDMCOUNT

[PATCH 5.4 077/191] MIPS: BPF: eBPF JIT: check for MIPS ISA compliance in Kconfig

[Greg Kroah-Hartman]

From: Alexander Lobakin <alobakin@dlink.ru>

commit f596cf0d8062cb5d0a4513a8b3afca318c13be10 upstream.

It is completely wrong to check for compile-time MIPS ISA revision in
the body of bpf_int_jit_compile() as it may lead to get MIPS JIT fully
omitted by the CC while the rest system will think that the JIT is
actually present and works [1].
We can check if the selected CPU really supports MIPS eBPF JIT at
configure time and avoid such situations when kernel can be built
without both JIT and interpreter, but with CONFIG_BPF_SYSCALL=y.

[1] https://lore.kernel.org/linux-mips/09d713a59665d745e21d021deeaebe0a@dlink.ru/

Fixes: 716850ab104d ("MIPS: eBPF: Initial eBPF support for MIPS32 architecture.")
Cc: <stable@vger.kernel.org> # v5.2+
Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Hassan Naveed <hnaveed@wavecomp.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/mips/Kconfig        |    2 +-
 arch/mips/net/ebpf_jit.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -46,7 +46,7 @@ config MIPS
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES
 	select HAVE_ASM_MODVERSIONS
-	select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
+	select HAVE_EBPF_JIT if 64BIT && !CPU_MICROMIPS && TARGET_ISA_REV >= 2
 	select HAVE_CONTEXT_TRACKING
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_C_RECORDMCOUNT
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c
@@ -1803,7 +1803,7 @@ struct bpf_prog *bpf_int_jit_compile(str
 	unsigned int image_size;
 	u8 *image_ptr;
 
-	if (!prog->jit_requested || MIPS_ISA_REV < 2)
+	if (!prog->jit_requested)
 		return prog;
 
 	tmp = bpf_jit_blind_constants(prog);

[PATCH 5.4 078/191] MIPS: Avoid VDSO ABI breakage due to global register variable

[Greg Kroah-Hartman]

From: Paul Burton <paulburton@kernel.org>

commit bbcc5672b0063b0e9d65dc8787a4f09c3b5bb5cc upstream.

Declaring __current_thread_info as a global register variable has the
effect of preventing GCC from saving & restoring its value in cases
where the ABI would typically do so.

To quote GCC documentation:

> If the register is a call-saved register, call ABI is affected: the
> register will not be restored in function epilogue sequences after the
> variable has been assigned. Therefore, functions cannot safely return
> to callers that assume standard ABI.

When our position independent VDSO is built for the n32 or n64 ABIs all
functions it exposes should be preserving the value of $gp/$28 for their
caller, but in the presence of the __current_thread_info global register
variable GCC stops doing so & simply clobbers $gp/$28 when calculating
the address of the GOT.

In cases where the VDSO returns success this problem will typically be
masked by the caller in libc returning & restoring $gp/$28 itself, but
that is by no means guaranteed. In cases where the VDSO returns an error
libc will typically contain a fallback path which will now fail
(typically with a bad memory access) if it attempts anything which
relies upon the value of $gp/$28 - eg. accessing anything via the GOT.

One fix for this would be to move the declaration of
__current_thread_info inside the current_thread_info() function,
demoting it from global register variable to local register variable &
avoiding inadvertently creating a non-standard calling ABI for the VDSO.
Unfortunately this causes issues for clang, which doesn't support local
register variables as pointed out by commit fe92da0f355e ("MIPS: Changed
current_thread_info() to an equivalent supported by both clang and GCC")
which introduced the global register variable before we had a VDSO to
worry about.

Instead, fix this by continuing to use the global register variable for
the kernel proper but declare __current_thread_info as a simple extern
variable when building the VDSO. It should never be referenced, and will
cause a link error if it is. This resolves the calling convention issue
for the VDSO without having any impact upon the build of the kernel
itself for either clang or gcc.

Signed-off-by: Paul Burton <paulburton@kernel.org>
Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
Reported-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Brauner <christian.brauner@canonical.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org> # v4.4+
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/mips/include/asm/thread_info.h |   20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

--- a/arch/mips/include/asm/thread_info.h
+++ b/arch/mips/include/asm/thread_info.h
@@ -49,8 +49,26 @@ struct thread_info {
 	.addr_limit	= KERNEL_DS,		\
 }
 
-/* How to get the thread information struct from C.  */
+/*
+ * A pointer to the struct thread_info for the currently executing thread is
+ * held in register $28/$gp.
+ *
+ * We declare __current_thread_info as a global register variable rather than a
+ * local register variable within current_thread_info() because clang doesn't
+ * support explicit local register variables.
+ *
+ * When building the VDSO we take care not to declare the global register
+ * variable because this causes GCC to not preserve the value of $28/$gp in
+ * functions that change its value (which is common in the PIC VDSO when
+ * accessing the GOT). Since the VDSO shouldn't be accessing
+ * __current_thread_info anyway we declare it extern in order to cause a link
+ * failure if it's referenced.
+ */
+#ifdef __VDSO__
+extern struct thread_info *__current_thread_info;
+#else
 register struct thread_info *__current_thread_info __asm__("$28");
+#endif
 
 static inline struct thread_info *current_thread_info(void)
 {