[PATCH 6/8] x86/fpu: Fix potential underflow in xstate_calculate_size()

[Andrei Vagin <avagin@google.com>]

xstate_calculate_size() calculates the size required for a given set of
xfeatures. It determines the topmost feature by finding the most
significant bit in xfeatures using fls64(xfeatures) - 1.

If xfeatures is 0, fls64(0) returns 0, and topmost becomes -1.
Previously, topmost was unsigned int, so -1 underflowed to UINT_MAX.
This caused the subsequent check `topmost <= XFEATURE_SSE` to fail, and
the code proceeded to access xstate arrays using topmost (UINT_MAX) as
an index, leading to an out-of-bounds access.

Fix this by checking if xfeatures only contains legacy features (FP/SSE)
or is empty (xfeatures <= XFEATURE_MASK_FPSSE) before calculating
topmost.

Fixes: d6d6d50f1e80 ("x86/fpu/xstate: Consolidate size calculations")
Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
---
 arch/x86/kernel/fpu/xstate.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a7b6524a9dea..ed39d7051d0d 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -589,12 +589,13 @@ static bool __init check_xstate_against_struct(int nr)
 
 static unsigned int xstate_calculate_size(u64 xfeatures, bool compacted)
 {
-	unsigned int topmost = fls64(xfeatures) -  1;
-	unsigned int offset, i;
+	unsigned int topmost, offset, i;
 
-	if (topmost <= XFEATURE_SSE)
+	if (xfeatures <= XFEATURE_MASK_FPSSE)
 		return sizeof(struct xregs_state);
 
+	topmost = fls64(xfeatures) -  1;
+
 	if (compacted) {
 		offset = xfeature_get_offset(xfeatures, topmost);
 	} else {
-- 
2.55.0.691.gc56d675ccc-goog